You sound stressed - don't worry you'll find a way to keep dancing. Put your energy into that.
You are asking about dieting and starting pointe, but those concerns are secondary to finding a good dance school/director. Once you have a great place to dance and good mentor, the rest will fall into place.
Where are you located? How far are the closest major cities?
Also, how old are you? When I was in high school, the "Gifted & Talented" advisor worked out a system where I would have excused absenses for dance. One of my friends got elective credits for leaving school early every day to take the train into another city to dance.
The second thing you should think about (not worry about) is your parents. Sometimes you have to follow your ambitions without their support. Find out what their major objections are and show the respect and maturity to consider what they are saying but ultimately make your own decision.
Good luck + don't forget to have fun