Is it permissible for a school to sell food items to female students in the school cafeteria, and then raise the price of some goods to benefit from the increase for the school's advantage?

The owner of the commodity has the right to increase its price as much as he wishes, as long as it does not constitute a monopoly. Since the questioner (sister) is entrusted with selling, and the school has increased the price of the commodity in a way it deems beneficial for the interest, then there is no harm in that.