My SIL teaches at a Catholic school but is not a nun. She is allowed to accept gifts from the students.
There are several nuns that work there as well. Because of Canon Law, the nuns turn over any money or gift cards that they receive as gifts to their Order.
That's going to be specific to each order of nuns. My aunt was an IHM nun, and we always gave her gift cards. Those she kept and could spend on herself (or her classroom or whatever). Cash she gave to the poor box. (I suspect "turn over to the order" isn't actually accurate - most likely the money goes to whatever charity/poor box/etc the order has set up)
If you know the teacher well, give her something you think she would like, just as you would any other teacher. If not, something generic like a target gift card would be great - she can spend it on herself or her classroom, depending on what she needs.
It's hard to remember nuns are people too! The summer after 7th grade some friends and I ran into our Principal and three of the nun-teachers from our school at an ice cream shop. We couldn't get over the fact that the nuns ate ice cream!
