Ok, I am going to play the devils advocate.
Why are you in such a hurry to get a snake? Are you one of these people that have to have instant gratification? What will you do when you are in collage? You said you will graduate at 15, so you must be smart. What is wrong with waiting a few years? You are living under your parents roof so you must follow their rules. Sorry but that is the fact of life. You will ALWAYS have to follow someone else's rules. If you get an appt are you going to go into a no pet place and 'sneak' in your pet? What if it gets out? Will you have a job to feed this snake? What if the snake gets sick and needs a vet? Who will have to pay for it? What about when you get a boyfriend and a job? Are you willing to take the time to care for your animal even though you want to spend time with him? What about studding?
Life is full of "I want that now" things. You see a LOT of people giving away animals, especially snakes and chinchilla's because they DO require care. Oh, it is fine at first but then the grind of the reality kicks in. You are sick and you just cleaned cages and your snake takes a dump in the water. You don't feel like cleaning it AGAIN. Are you going to let it sit or grit your teeth and clean it?
I am sorry to be such a stick in the mud but I have seen what happens when a young person gets a "long term" pet. Pets are a handicap when looking for a place to live as well make no bones about it (unless you plan on living with mom and dad for the next 10 years or so). Especially snakes. A LOT of people are afraid of them. When they ask about pets are you not going to say anything or speak up? Let me tell you when the word "snake" leaves your lips, the landlord will find SOME reason not to rent to you. What then?
Also you already have pets, why not just be happy with what you have?
LOL Ok I am going to get off my soap box but just some thoughts from a mom that has been where your mom IS.