Cant pray dhuhr, asr and magrib due to school?

Assalamu alaikum, in winter time i cant pray dhuhr, asr and magrib because I'm at school at this time. I know i can repray it and i do that, but is it a sin or do i have to pray it at school?? Because i live in germany, in a non-muslim school and my teachers arent very openminded. So what can i do? is it ok to repray 3 prayers if u couldnt do them?? I have tried to ask one of my teachers but she didn't allow me to do so. Please reply me.