During the Osaka arc of the anime, Noboru saves the girl who made his life hell over the last few years, TWICE. once by diving down the hole to prevent her from drowning and then preventing her from getting sandwiched by a jet of water in the cave. And why did he save her? "Because it was the right thing to do".
One of the best Crowning Moments of Heartwarming of all time happens in episode 11 of the anime, where Tomoko delivers a soliloquy on friendship. If you don't feel anything after hearing her speech, quite frankly you're dead inside.
Noboro and Anko during their time in the cave
The picture that Urumi put behind a massive firewall was a picture of a young Miyabi, Tomoko and Urumi as friends.
After Miyabi revealed to her classmates that she had framed their former teacher for sexual harrassment and unintentionally started their hate for all teachers and was rushed to the hospital after a fall, Onizuka (who ended up in the hosptal with her) brings her outside where everyone in her class was sleeping to make sure she was all right.
When Tomoko plays house after school and is all lonely and sad, and Onizuka joins her with his own character like it's nothing.