Can you ask to do different chores around the house to make extra money to pay for it? Wash their cars, vacuum the living room, etc. That will show you are responsible. Then tell them some of the reasons are:
1) They can get a hold of you at all times to know where you are.
2) You won't have to mooch off other friends to borrow their phone to call home and ask to go to Suzie's house with Vanessa.
3) You can prove you are responsible by NOT losing the phone or jumping in the pool with it.
4) Your friends have one and you just want to fit in.
5) In case of an emergency you can call 911 and them for help.
6) Since you have good grades, it's a reward and privilege that you've worked towards and ....if the grades slip the phone can be forfeited until the grades come up.
7) You can improve your math skills by calculating the minutes you've talked each month.
8) You can call your dad just to say, "I love you!" from anywhere anytime.
Good luck!