Look you go to a private school, they can ban the color orange if they want to, if they don't want beards than thats their decision. Most likely it is aimed at pepole who would not take care of their beard and let it look bad, the school has an investment in how the students look after all.
I go to a private school to, we are not allowed to have facial hair there either. However 3 students have beards based on religious grounds, Eastern Orthodox Catholics. Same thing with hats, some students wear yarmulkes, obviously Jewish, and that is perfectly fine. At my school small amounts of well kept facial hair are generally ignored unless their are pepole visiting.