How To Convince My Parents To Let Me Do Tae Kwon Do?

I am now finishing my first year in high school. I got all A's this year and have a 4.125. However I am stressed and would like to do Tae Kwon Do. I've been wanting to do it for 2 years but my parents aren't sure about it. I found a place 5 minutes from our house that is 100 a month with a free uniform(its a legit place). I run cross country 3 months a year but I'lI need something to do the other 9 months. Especially over the summer, I get very bored. I don't want a cell phone like they offered for my all A's I just want to do Tae Kwon Do. How can I ask?