In my Honest OP I think you need to reevaluate your life. The best at BMX or Not you need to be flexible in your goals in order to succeed. Your parents are being a bit tough on you, but pretend your them for a minute. They see their son failing school and just riding around on a bmx, as worried as they are they see you went out and even broke your leg.
Their your parents. They in their heart want whats best for you. Try to compromise in a way that makes you and them happy. Tell them that if you promise them good grades will they support you in your bmxing? you need to make time for your school work and not just bmx all the time. Just try to fit school into your life, dont come home so late you get bad grades. Everything in excess is bad. find a balance and youll see everything will turn for the better.
First things first. Talk to your parents and tell them that you are willing to make time for school if they make time to support your BMX. Keep your promise and make sure they keep thiers.
God Bless.