@pounce yeah I see the issue. Usually, in situations like these, if one of the party uses "tu", it means they're okay with you using it back. It happens often enough in fact, but people usually just adjust to each other on the go.

And as for this context, a classmate is someone on equal term to you, right? Close in terms of age and the fact you're in the same group (or here, class). You can take a casual approach. For a teacher? You might wanna show a bit of respect, as they're older and the one in charge, and use "vous" instead