What does it mean when a guy kinda ignores you in school but outside of school when you see him he flirts with you and over text says really random things and we're both average ish popular?

It means he doesn't want his friends to know he likes you :) And he doesn't want to embarrass himself from a certain someone and maybe he does like you but doing it slowly:)

it could also mean hes worried about you telling other people, so he does it while your alone, and if your brave enough maybe say somthing like "your alot more friendly outside of school" and maybe he'll get the hint and tell you he wants to be more than friends