did you ask her why?
breeding snakes is quite expensive, so it's the first reason i can think of. another reason is it can take a lot of your time, time which you should be spending on schoolwork. maybe you can bargain with your mom. snake breeding expenses to be deducted from your allowance or you'll work for snake money, or promise you'll excell in school, or all of the above to make the proposition irresistable. to show you are sincere, you can make a headstart even before your mom let's you get the snake.
i'm concerned about the "or something else" part. the grown up thing to do is settle the manner reasonably. going behind your mom's back is never a good idea, no matter how right you think you are.I would just like some help on either convincing her, or something else.