User blog comment:Warmachine375/Where was Team CFVY when Velvet was being bullied by Cardin Winchester and his gang of bullies?/@comment-3257939-20150527003810

Simple answer: College.

They're seventeen years old kids. That's the most common age to start college. Since there are other minor schools like Signal, it means that Beacon Academy is like College for Hunters where they're not only trained but also work as hunters.

Team RWBY doesn't share all their classes despite being in the same semester and the same team. If Miles and Kerry really have to explain it, they'll most likely say that Velvet simply has a different schedule so she eats lunch without her team.

A better question would be: Why is Cardin still doing it? He's bullying the member of one of the most (apparently) famous teams in Beacon Academy. All the other members look very intimidating. He knows that Velvet only needs to say the word and her team will wipe the floor with them. Maybe it's because Velvet is against unnecessary violence and Cardin is aware of this so he knows they won't fight back unless Velvet says so.