I think if you can get a locking cage, show your mother you've done your research as well as joined a great forum where you will get tons of support and information, that you are prepared to cover any costs for food, housing, vet, etc. and that you have a plan for the years ahead (for instance if you go to college, what happens to the snake?)...that sort of thing. Parents are always more willing to listen to their kids when those kids have a well thought out plan rather than just "I want this!" sort of thing.