Just to expand on an idea and correct the secrecy of magic thing, why not have Sakura from CCS be just a regular girl from class? Throw in a Pactio and she gets the book and wand. Much like Nodoka's artifact shows, the weapons can be improved through training. Start her off with some moderately weak cards, and as she increases her proficiency she gets more.
To throw my own idea in, Louise from Zero No Tsukaima. Could probably be trained by Albireo, considering his particular brand of magic.
Not a girl, but Hachibe from Ai Kora would make a good teaching aide. Especially when he tries teaching Negi the glory of Parts Love! Could make good comic relief, if nothing else.