Well Christian, it's like this, we can't help give you ways to convince your Mom to let you have another snake, you know your Mom best.

How has your track record been as far as responsibility?
Are you a good student?
How many pets are in the house now?
Does she even like snakes/reptiles?
Does your Mom even like pets of any kind?

I was fortunate enough to have a Mom that let us keep a variety of pets, as long as we took care of them. I was the same way with my children, who had the habit of bringing home every stray/injured animal they came across.