On

ON time is correct 'In' our class sounds slightly strange - we are more likely to hear, 'TO' our class. Do you mean, during the class we must be on time? (In which case, IN is ok) Or, do you mean, we must be on time arriving to our class? (In which case, it should be TO).

Unless the sentence is reorganized, “on” is the preposition needed.

Hi

In