I have pretty much the same situation as you (16, female) and I got my first corn snake a couple months ago. Getting her all set up ended up costing around $135 IIRC, so make sure your mom's willing to spot you a bit if you're buying the whole set-up on $60. Although, my corn was really overpriced and I had to buy the tank and the fun cage decorations and all that jazz.
Anyways, I think corns are excellent pets. I never had a problem with time restrictions, because Satchmo would usually prefer to be left alone, if anything. (That's not to say she wasn't friendly when she came out.) I worked her feeding schedule in with my schedule, feeding her Sunday nights and not handling her Monday or Tuesday because I was busy with music lessons and band rehearsals, respectively. Then I could take her out and show her off the rest of the week.
Does your mom know about the mice-icles taking up residence in the freezer? I put mine into a tupperware container and you couldn't even see them in there, but I know some people can't deal with the thought of little rodents hanging out next to the pizza rolls.
Before you buy, make sure you research a bunch. There's caresheets everywhere online and you can browse these forums. A good book for beginners is The Cornsnake Manual by Kathy Love. I got a copy at Petsmart for $10 but you can order online too.