Hey, CarmenCarminis. . Withdrawing from the offer you committed at first does not constitute the violation of any law, in my opinion. And I guess the school will probably release you because they do not wanna someone who does not consider him/herself belonging there. You do have some risks, although not big, burning the bridges to your dream school if your first choice does not want to let you go. But again, the risks are quite low.
However, it is best to ask for an extension. Tell them your situation, be honest and sincere. They all know that students have many competing offers.