Wikipedia says that Max is 11. I don't know if that means he was 11 in the first season and is now older than that. Probably not, since he would be in high school if he was 13. I would assume he's around grade 6-7.
Fun trivia, the actor who plays Max is also named Max, and the actor is actually 14.
As for your other questions:
1) I think the classmate did ignore him, because he already thought Max was weird and didn't want to talk to him.
2) I don't remember that part of the episode well enough to answer.
3) Yes, though Jabbar is 5 or 6, so the friends he made were quite a bit younger than him.
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