How to convince your parents to go horseback riding?

i really want to go horseback riding but my parents think it's very danerous and no exercise whatsoever. I have been to a barn before and I have been around horses. My parents already paid for a year of hockey, but I am talking about just for the summer maybe. My teacher said I am doing very well in school. School ends June 30th so should I take my good grades to consideration for riding during the summer? I want to get straight A's. Please help me