Your manager can ask for more hours. You can answer honestly about what you can handle.

A request for more hours is a request, and honesty is a better answer than willingness you cannot sustain. Managers are solving a rota problem and would generally rather know your real limit than discover it in three weeks when you are exhausted and unreliable. Saying yes to everything looks like keenness and ends in dropped shifts. Accurate is more useful to them than eager.

Work out what you can actually do alongside school and say that number. I can do two evenings and one weekend day. Offer the specific version rather than a vague not much, because specifics are easy to schedule around.