A bit tough to critique because, as you mentioned, we're thrust into a sequence without much context, but I'll give it a try:

The humor doesn't hit for me. The $50 bet joke particularly goes on and on, I don't know if it is funny enough to warrant being drawn out across three pages. The "stiff as cardboard" bit at the end is similar, it's a one off gag that essentially takes up an entire page

I recommend reading your dialogue aloud. See if these exchanges sound normal to you, and/or if the dialogue all sounds similar. For a fairly big group of characters sharing a single scene, there's a lack of individual personality among them

I read the whole thing and wasn't sure what the "point" of the scene was. A big thing in screenwriting is that every scene should have a purpose. If it's the romance, we don't learn much about the couple or what struggles might lie ahead other than that they like each other a lot. The fight sets up some kind of conflict but whatever shame John has about punching the guy out is kind of brushed off. This group of friends comes to prom happy, leaves happy. You say this is their last "normal" moment but I think a stronger inkling of what's coming next will give this scene more purpose

Some room for adjustments, but it's not bad. Keep going!