In the school PS.261 if you are late you will get detention because many people have been coming to school late. The reason why the principal wants us to come in early is because we are missing a lot of lessons in our classes. So the next time your teacher asks you about lessons you may have missed for being late, you'll understand it.
If you take the bus it doesn't matter if you are late as it is not your fault, it is the bus driver's.
He is one tip to be early, come earlier than you usually do.