If you are in a location where your identity must be confirmed, you cannot wear something that prevents the confirmation of your identity. This should be true pretty much anywhere... banks, schools, your workplace, etc. You should not be able to use religion as an excuse to get away with behavior that is against the rules. The teacher needs to know that the person being graded is the person taking the test, the teller need to know that the person withdrawing money is the person that owns the bank account etc.