F.,
I don't know if this will be helpful to you or not but when my son was in 3rd grade I had a baby and they gave me a baby shower in the classroom. They had assigned each one of the children a letter and ran off a coloring page onto some type of iron-on material. Each child colored his page (they stressed heavily to the children to do their best work, but would not tell them why they were making these, so as not to ruin the suprise) The classroom aide then transferred the pages onto material and made the most adorable ABC quilt for the new baby.
Maybe you could incorporate this idea but gear it more towards a teacher type theme? I'm sure it would be a very sentimental thing. Maybe a photo of each student made into quilting squares?
If you are in the classroom often as a helper, maybe you could get someone else at the school to conspire with you to discreetly get the kids help. You may be able to make this a shower activity for the kids if its not going to be a suprise for the teacher, and the quilt could be given at a later date.
If your not the crafty type you could always have a pantry shower and each student could bring in something to help stock the bride to be's food cupboard.
As for shower food, I would keep it to cake or cupcakes and punch. Shower activities could be as simple as a wedding themed word search, or even a Mad Libs page. Or you could have each child make a wedding card for the teacher with well wishes in his or her own words and then seal them up to be opened by the teacher after she is married. Good luck it sounds like a very thoughtful yet fun thing for you to do! Good luck.