Chiming in as a mom here...
At the age of 13, the last thing you want to be doing is sneaking around and doing things that will cause your mom not to trust you. If I caught my 13 year old son with a snake (or any animal) that I hadn't given him permission to have, I would make him re-home it and my trust would be shattered. You have your whole adult life to have as many animals as you wish. Your mom is kind to let you have one snake (many moms would say no way). Work with her and talk with her if you want another. Strive to be the trustworthy son that I'm sure she wants to raise. Your long term relationship with your mother is much more important than having another snake at this point.