Do what I did: make a pros and cons list of having a phone and show it to your parents, and have a serious conversation with them. Tell them that you're getting older and you deserve a phone. I assume you go out to places with friends, so tell your parents that you may need to call them if an emergency happens and you need them to pick you up, or if there's an emergency at school and you need them to pick you up right away.
I didn't get a phone until the eighth grade because my parents didn't think I needed one, but I ended up getting one for Christmas that same year. For me it was harder to convince them to get me a phone since I was in junior high, but it should be easier for you to since you're already in high school.
If you're parents are THAT difficult to convince, tell them that you'll pay for half of it if they pay the other half. Compromise with them.