Can my manger ask me to choose between school and work?

Manager now a day think they have more power than the boss

Yes. When you take a job, you need to commit to that job. If your schoolwork interferes with your job performance, your employer can ask you to make that choice.

I merged your two threads, Please don't start multiple threads with the same issue.

Your manager IS your boss. If you feel your manager is overstepping his authoritiy then you go to HIS boss to discuss it.