Seeing the kids start to light up as Dewey connects with them through music and infects them with his actual passion. Even Summer starts tapping along.
Dewey has a small Heel Realization when one of the kids sadly asks, "You mean we're not in the band?" and tells him that "Just because you're not in the band doesn't mean you're not in the band." and starts assigning everyone positions.
After seeing Zack berated by his father in the parking lot, Dewey initiates a class lecture/improvised song about sticking up for yourself. By the end, Zack is feeling much better and even thanks Dewey for the great lesson... in front of the other teachers (who are both awed and slightly jealous).
Dewey's little speech to Tomika when she worries about her weight. He lists Aretha Franklin, who he knows Tomika respects, as a Big Beautiful Woman who's also a fantastic singer.
Also, when Tomika worries that she can't be taken seriously because she's fat, Dewey doesn't say the common backhanded compliment "You're not fat, you're beautiful!" He tells Tomika that it's entirely possible for fat people to be both beautiful and talented.
When Freddy goes off to play cards (and drink?) with some douchebags, Dewey is terrified. He lost a kid and found him with some jerks who are willing to daydrink with a ten year old. He rips them a new one and tells them they have to be responsible when working around children. He's finally learning responsibility himself.
Any and every time Dewey bonds with one of the kids. Especially the way he interacts with Lawrence and Tomika and helps them deal with their respective self-esteem issues. It really shows that Dewey is a Cool Teacher at heart.
It starts at lunch right after Dewey introduces the kids to rock. Larry says he doesn't think he's cool enough to be in the band, but Dewey tells him that being in the band is what makes you cool enough to be in the band and teaches him lame cool handshake.
A small but sweet moment: when Billy asks Dewey to be the stylist, Dewey immediately tells him "of course you can, Fancy Pants". While this is obviously intended to be a gag playing on Camp Gay stereotypes, Dewey's reaction In-Universe is completely understanding without any trace of exasperation.
As noted here, Dewey doesn't gatekeep the kids or shame them for not knowing who Pink Floyd or any of the classic rock musicians are; he just enthusiastically shares his own passion with them in the hopes they'll become fans too.
The fact that Dewey ensures he includes the entire class in some way or another, even the ones who don't have any musical talent or training. It would have been easy for him to simply focus on the core band members and leave it at that, but Dewey goes out of his way to give every kid a job to do and even ensures they know that their roles are important whether they're on stage or not.
Summer comes up with the idea to tell the judges the kids are all terminal as a way to get them into Battle of the Bands - after Freddie makes them miss the audition. After Dewey tells her she's getting an A, she smiles and says "I didn't do it for the grade".
Ros getting a chance to let her hair down in the bar with Dewey. She says that none of the teachers have ever asked her to get drinks before.
She also later asks him to be there at Parents' Night because she needs a friend in her corner. D'aww...
While it is partially to get the go-ahead on the school trip, Dewey is genuinely delighted at seeing her more wild side and sympathises with the difficulty of her job and the precarious position she's always in.
Upon Summer telling Dewey that Freddy went off to hang out with a shady band, Dewey immediately goes full Papa Wolf and hurries to find him.
It has devastating consequences, and Dewey tells off Patty because it hurts the kids, but part of the reason Patty called the cops on Dewey for impersonating Ned was that she rightly told him off for stealing Ned's identity and discrediting his teacher reputation.
After Dewey is proved as a fraud, the kids still insist on playing in battle of the bands and even go to pick him up.
During Parents' Evening, Dewey sincerely compliments the kids.
When he calls Zack the next Jimi Hendrix, what appears to be his then still unsupportive father strokes his head.
The Battle of the Bands is one heartwarming moment after another:
The plan was for the band to sing Dewey's song - but when they get there, he changes things up so they're singing Zack's song, which he freely admits is better than his and more representative of the band. The way Zack's face lights up as he's told this...
When Tomika sings her solo at the Battle of the Bands, it cuts to her parents gasping with delight in the most heartwarming way. Her mother looks like she wants to cry Tears of Joy.
When Dewey says that Zack wrote the song, his father is floored, and that's before they start playing! A short while later, Zack's father is cheering him on.
Lawrence's father: Your son's very skilled. Zack's father: Thanks. So's yours.
There's also the parents and Ros really popping when the singers sing "kick some ass" in the performance.
After the song ends, Frankie and Leonard share a high-five in the wings.
Ned cheering for Dewey moments later.
After their performance, one of the members of No Vacancy congratulates Zack on their song, genuinely being extremely encouraging of a young musician finding his voice. Another is impressed with Freddy's drumming.
Ros rushing up and hugging Dewey, absolutely awestruck by the performance. She admits she's pretty angry about the whole ordeal, but she's too proud of him and the kids to care by that point.
The context of Zack's song (titled just "School of Rock") is especially heartwarming when you read between the lines. Zack is so awestruck and grateful for Dewey's help as a teacher that he wrote an entire song out of gratitude, the lyrics talking about a "magic man" who taught the class rebellion and self-worth. The other kids don't even disagree with him or make fun of him, they have fun performing the song and share that statement about Dewey. Tomika even threw in her own verse and sang in the style of Aretha Franklin, someone Dewey referenced positively when he was giving her a pep talk.
As pointed out by the comments under this video of the Battle of the Bands performance, there's a layer of sweet irony in Dewey's choice of song to perform. "Legend of the Rent", Dewey's self-penned track which he intended to be the one played, is a cheesy "I Am Great!" Song ego trip where he complains about his mistreatment by his former bandmates. After his Character Development, Dewey is humble enough to choose Zack's song as the better one. In a roundabout way, Dewey still ends up singing an "I Am Great!" Song, only this time it represents the gratitude Zack and the kids have in him.
In the closing credits, the girls are a little confused by Jack Black scatting, but then they figure it out and laugh and sing.
Unique to the musical
The utter joy on Zach and Freddy's faces when Dewey tells them they are awesome at rock and roll and "in the band".
The first moment that denotes Dewey's Character Development is when Lawrence says he can't be in the band because he's "not cool enough." Dewey goes Papa Wolf and tells him he is cool. It wasn't just ulterior motive; he wanted a kid to feel good about himself.
Ned and Dewey playing Guitar Hero together. They really are friends, and we get to see Ned's rock passions.
While the first "If Only You Would Listen" is a Tear Jerker, we see that Tomika's dads really want the best for her and are oblivious at worse.
Dewey at first only takes Ms. Mullins out for a drink to get her to sign off on the Battle of the Bands "field trip". Then they start to bond when she sings about the girl she used to be. By the end of the show, she forgives him for lying to her and committing fraud and covers for him when Patti summons a police officer. The parents and she agree that he can be their kids' afterschool rock instructor, under his real name. They then kiss, and she shows up for the Curtain Call in a rocker chic jacket.
Dewey gets a Heel Realization when Ned angrily tells him that the impersonation could have cost Ned his teaching license and their apartment.
The kids' Triumphant Reprise of "If You Only You Would Listen," which they use as a Pep-Talk Song for Dewey after he's outed as a fraud and is lying in his apartment in Heroic BSoD. They tell him that he was the first adult that actually listened to him, and that's why they want to play with him.
Unlike the kids comforting Dewey in the film about losing to No Vacancy, Dewey comforts them. He wanted to win the Battle of the Bands to spite his old band and get the rent money, but he doesn't care about that anymore; he cares about his students and tells them they did a great job.
Ned in his rocker outfit sweeps Patti off her feet, literally, and says he's treating her to Red Lobster. It's implied Patti gets a Heel–Face Turn and mellows towards Ned and Dewey because of it.
, Dewey doesn't gatekeep the kids or shame them for not knowing who Pink Floyd or any of the classic rock musicians are; he just enthusiastically shares his own passion with them in the hopes they'll become fans too.