Using a local flower shop? To surprise her, I don’t want to ask her address. She is only there visiting for 6 weeks.
Your thought is charming, but not practical. In order to deliver the flowers, the delivery person would have to ask what class the student is in, and people in the front office do not give out that information except in an emergency. In the rare case that someone in the office goofs and gives directions, the delivery person would have to interrupt a class in order to make the delivery. This irritates the teacher and embarrasses the young lady. Is that the reaction you want? And finally, what will she do with the flowers until school is over? Will they fit in her locker? Should she carry them around with her all day? And she already has armloads of stuff to carry home, so the flowers become a nuisance there as well. If you want to attract her attention, you are better off biting the bullet, asking for her address, and sending the flowers to the place where she is living. (You could also try walking up to her and conversing.) The flowers idea works great in movies, but in real life, not so much.