One things about teens and snakes (I have both LOL) is to make a plan for her future as a snakekeeper. Most of the snakes recommended, with good care, could be her scaley buddy for up to 15 or 20 years or better so a plan for what will happen to her snake if she attends college and can't take it with her is a good thing to discuss up front.
I really like mainbutter's suggestion of bringing her into the discussion. I would encourage you to print up a gift certificate type of thing from you saying you will cover the cost of the snake, it's enclosure and 6 months of feeder prey or something like that. She should be encouraged of course at her age to assume responsibility for it's monthly costs at some point. I'd also encourage her to join this site so she can learn and find support in a family friendly environment.
I'd also encourage you to consider a hatchling from a good breeder over running down to your local pet store. Most pet stores, unless you are blessed to live near a VERY good one, won't even know the sex of the snake, when it was born or hatched, and sadly often sell you a snake with a free case of snake mites. If you happen to live in an area with a good monthly reptile show that's also a purchasing option. Once your daughter does decide on what snake species best fits her, we'd be happy to help you find a good show or good breeder.