Hi Jack,
If the folks are okay with it and the therapist and your doctor give the thumbs up, then your good to go. Your parents may be okay with it for now because it is just talk at this point. Are you living outside the house as a girl? I know from personal experience that a boy going about dressed as a girl when everyone knows they're a boy can result in the sudden need to move. Even a teen transexual who is actively transitioning can cause great upheaval in the lives of their family friends and loved ones. Are your parents aware of this factor and are they prepared to deal with it? Also before and during hormone therapy you will be required to live full time as a girl. This means at school as well as home.
Are you prepared for that and what that entails? The school will have to accomadate your special needs. School boards can be very obstinate and may not even allow you to come to school as a girl, regardless of what any doctor says. If you are taking hormones at your age the effects could be very rapid and profound. If you are forced by the school into the male areas, i.e. restrooms, locker rooms, etc. this will be a great hardship you might have to be pulled from school and your parents forced to find a cooperative school elsewhere. They may have to relocate to find one, change jobs, sell the house to buy one in the TS friendly area. Do you still think your parents would be okay with it under those circumstances.
There are a lot of issues here. There are a lot of things you will have to work out. You're seeing a therapist and that is good. Is the therapist experienced in handling transgender youth, or transgender issues of any kind? I think legal issues are the least of your problems. If you are determined any obstacle can be overcome. I am not trying to discourage you or rain on your parade I'm just pointing out a few facts you may not have thought of.
Good Journey,
Cassie