How to convince my dad to let me have an iPhone 4S?

[Vulgarsugar306]

For the longest time I've wanted an iphone, but my dad thinks people of my age don't need smart phones, and my mom thinks it's too expensive which doesn't make sense because the allowance I get every month is enough to pay for the whole bill. I even went on AT&T's website and saw what plans would best suit me, and it came out to $90 a month, which I can pay for on my own. I'll even buy the device and cases etc. I understand that my dad thanks I'm just a teenage girl (14) who will spend all her money once she sees clothes she likes, but I wouldn't do that if I had the responsibility of the phone. Basically how can I convince them athat I can handle/pay for the phone.. I've seriously considered writing an essay or something like that lol....