How can I convince my parents to purchase me a cell phone.?

Money, grades, and home are not an issue in my situation. I have saved up about two-hundred forty dollars to buy myself a prepaid cell phone just to start out. Im 14 years old and I'm the only one in my entire and i mean entire school without a cell phone. 6th-8th grade. I kinda feel left out and bummed out. I have shown my responsibility by bringing home great grades and walking the dog and etc. They are in denial of me getting a cell phone. I've only mentioned this topic to them only 3 times and they got fed up and grounded me. I always go to the mall or at my friend's house and can't get a ride all the time because I have to use my friend's phone all the time. It's so embarring! I do after school activities and I have to walk home from school everyday. Im going to be a freshman and my parents said not until your 17. What the hell?! I know I have the money, but if they catch me that I bought a phone I'll face dire consequences. My High school is too far from our house for me to walk home and If I miss the bus then im stuck there. pre paid and contract are my only options. I don't use the home phone because when ever someone calls It crackles and I dint hear the person at all. My mom has an android powered phone and my dad has the Iphone 4. I get teased and laughed at during the school year and just took it from everyone until the very end. My social life has taken a huge hit when I went from 5th grade to 6th grade because most parents got their kids a phone. I asked my parents why they haven't got me one yet and they replied back that wait until your 7th grade. I said ok and waited but surprise surprise I didn't get it. This lie has went on for three damn years. My plan for me is to intentionally get lost and get found so I can tell my parents. I just don't want a cell phone it's a necessity for me to get transportation and socialize. I'm in a situation where I can't buy a prepaid or contract phone.