The entire time, the other boys are kinda on her side, they see how she's feeling and that she doesn't wanna go to this event or be involved. And Mikey's all in to be encouraging but mostly because he's here for the drama, and is trying to be encouraging for the same reason Raph is. Leo and Don aren't really sure they should be involved and are just like 'Go, show your face, get your grade and bounce? What's the big deal?' Because they're both a lot more introverted in general, and overall-- I don't think Leo and Donnie really have that same longing to be part of the human world that Mikey and Raph do.
The Sunset Bros are being encouraging because they want to help Harley make friends outside of them, because they think she's cool and deserves to have more people appreciate it (aka to get made fun of and shunned at school less). Raph encourages her to go, and rationalizes it might be fun. Getting to talk to her classmates, do something not homework or class related for a bit, cut loose and be herself around them, and that she might be surprised by what she finds with that.
That's what he's hoping for, because he knows deep down its unattainable for himself. Harley is still hesitant, but she wants to believe in THEIR belief of her, and so ends up going, there's some secret ninja shenanigans. I hadn't had a chance to map it out the other day cuz at the time i was at work, but maybe the seniors or some trouble makers are trying to pull a prank at the dance, and it's the boys-- sneaking and following Harley as encouragement to make sure it goes well all look at eachother and go
Leo: 'Oh man-- if those seniors manage to pull off this prank it'll be a disaster.'
Raph: 'The whole school dance'll be a bust.'
Mikey: 'And then Harley will be super upset we made her come here and she'll never want to socialize again and become an antisocial hermit-wizard!'
Donnie: '.....Like Gandalf?'
Mikey: 'EXACTLY! We can't let that happen! We have to stop those senior citizen creeps!'
But the prank sorta ends up happening anyway, with Harley and the boys helping to stop it 'secretly' and it's a fun shenanigan episode of them trying to thwart the seniors while not getting caught or spotted and it's a very silly kind of situation, and in the end the prank kinda happens? but in a cool way, like maybe the seniors were gonna blast the whole gymnasium with glitter cannons or something and they manage to make it go off but instead of ruining everything it's not as explosive and it ends up being really cool looking and Harley impresses a few kids in her class, maybe the seniors themselves, Idk i havent fully fleshed it out~ That's all I got so far!