how can I make middle schoolers...sound like middle schooler

Well I'd say, me being a middle schooler myself, that we don't use words like bummer or that sucks. It's more vulguar language and joking about serious topics. I say myself(and I can't curse on this site because I got warnings) things like "well that sucks ***", and things like that. And when someone's talking about a teacher they hate they'll use something like(and I'll make an example):
"I hate Mrs. Andrews. She's such a *****," grumbles James, tapping his pencil against the cheap wood desk.

But that's only if you want to put 16 plus warnings on your story so yeah... lol
and also sometimes people refer to each other as "bruh" not bro though, and for greetings I've heard people say "ay man wassup". No one really does fistbumps anymore, more like their own type of handshake or just a handshake but not an actual handshake, more of a clasping the other person's hand and then pulling both of their hands downward slightly.

this is supposed to be clean so no curse words. it might come off as stilted but I really feel like I just don't want to do it.

Well, there are a lot of ways around that like freaking, crap, etc.

use of slang. Doesn't even have to make sense, as long as its a replacement for something else. You can have kids running around saying 'crunchy' as opposed to 'good' and people will accept it.

Complaining. All 13 year olds do is complain about EVERYTHING.

Diversity is still a thing, not all kids act and sound the same. Use that to your advantage. There's always one who acts like the cool kid, one who does 'that weird thing', one who is down on themselves, one with no confidence, one who's always in trouble, and girls are always fawning over this boy or that and changing their minds rather quickly.

MISTAKES! kids make mistakes. whether it be with english, old sayings, bad advice, or just ignorance in the face of whatever. Making mistakes is what growing up is all about. Misconceptions are a great method of showing age as well. Not knowing something, being afraid of something, or having a misunderstanding of how something works (that your average adult would understand completely) definitely gives a feeling for one's youth.

I don't think you need to focus too much on dialogue as a whole, so much as setting up personality. Some kids can be sophisticated, while others lacking. It's about giving the reader the feeling that they're younger, and you can do that through setting so much more than you can through basic dialogue (in my opinion).
Hope that helps.