You could place stepping stones throughout the garden that are made by the students. They could place their hand imprints, statements she used to say, or anything they would like in the concrete. Then you could plant a tree, have a small pond or waterfall and plant some flowers (perennials) that are the school colors. Also you could place a bench in the garden. I don't know how much room you have, maybe find someone who's in high school now that had her as a teacher and get them to make the bench during wood shop.
A couple of years ago our K teacher pasted away and we are looking for an idea for a momerial garden . we would like to put something in honor of her in this garden, she was a teacher in our school for 37 years and dedicated her life to her students. this is my last year at this school, my son will be moving on to the middle school, and it is very important to me to have this done before we leave the school, this teacher was very dear to me. Any ideas would be greatly apprciated. Thanks