What happens when Stargirl comes over to the table where Leo and Kevin are sitting?

Are you referring to chapter 18?

Stargirl and Leo are in love. They are so focused on each other that it takes Leo a few days to realize they are being shunned. No one speaks to them or even nods their way. They are so ignored that Leo tells Kevin he feels invisible. Kevin tells Leo that it is not he, but his association with Stargirl that is causing this, that Stargirl is getting "the silent treatment." Leo tries to defend Stargirl asking why the good things she does doesn't count for something.

Leo grows paranoid and is very aware of the cold treatment he is getting from others. Stargirl doesn't seem to notice.