This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Recess: School's Out
When TJ is riding his bike through town while his friends are all at camp. But what makes sad is the fact that the song "One" by Three Dog Night is played, and the lyrics are not cheerful.
Principal Prickly's The Reason You Whomp Speech to TJ, when he reveals that he genuinely does care about TJ and his friends, since they're his only link to his memories of his lost childhood innocence.
Prickly: I'll let you in on a little secret, Detweiller. Every adult you've ever known was a kid at sometime in his life. You think we don't remember summer vacation? Riding our bikes down the creek. Catching polliwogs in a jar. Camping out under the stars. Well you're wrong! Sometimes I sit there in my office, looking out at you kids in the playground and I think, "They don't know how good they got it. In a few years, they'll be grownups like me and all those good times will be memories for them, too". So go ahead. Put a whoopie-cushion in my chair. Put fake vomit on my carpet. Make fun of my "big, saggy butt". But don't ever say I don't care about summer vacation,(voice breaking) 'cause those memories are the last part of childhood I got left.
This really shows Principal Prickly off for who he really is. Rather than a generic kids show villain like most cartoons with a school setting will make the Principal, Prickly is a human being like everyone else, who had to learn to grow up and deal with the responsibilities and complexities of adulthood, something T.J. and company don't have to worry about yet. Its even more powerful when you find out Prickly's background, that he was an idealists teacher during the Summer of Love in the 60s, but all that idealism was eventually ground away as he aged, as he was forced to combat the mind-numbing bureaucracy of the educational system and the realities of the world. Prickly isn't a villain, he never was one at all, it just seemed that way to the main characters, who are children, too young to understand just what adulthood can do to you. Thankfully, he's set back down the right path by the end of the movie.
When the gang are exchanging stories about their favorite summer memories together, Gus straight-up bursts into tears—since he realizes that, being "the new kid" (with the gang the first consistent group of friends he's ever had) he'll never get to have any childhood memories that sweet. A surprisingly honest display of what it can really feel like to be the perpetually lonely kid in Elementary School.
Spinelli: What's the matter with you? You just moved here!
Gus: I know! And now I'll never share any of these memories!
The underlying regret in TJ's voice when Mikey has brought up "John Jacob Jingleheimer Schmidt" as "that song TJ's sister taught us that first summer after kindergarten" and he responds with "Oh yeah; back when she was nice." Even if he doesn't openly show it, it's obvious he misses the way he and Becky used to be.
The very last shot in the movie, of the gang relaxing by the pond after all the events of the film. It might not seem much, but the film was also suppossed to be the "official" ending of the seriesnote Although it was renewed for another season, season six was actually made up of leftover episodes of season five that hadn't aired yet, and pretty much a "goodbye" to the gang (until Recess: Taking the Fifth Grade was released).