I'd say any kind of decoration would be great...but as a teacher (and especially when I was a student teacher) the most meaningful thing to me is (was) notes from students. In all their misspellings, strange grammar, and such--their words are the most meaningful and encouraging. Maybe have them write and draw something, then put up the pictures and notes, then have a binder ready to put all the sentiments in so she can take it with her. (A binder makes it easy to take the memories with her, wherever she goes.)
A big banner with an appropriate message and all the children's signatures to put across the blackboard might work. I have seen these with magnets glued to the back and the magnets then hold it to the metal stripping around the blackboard. The banner would make a very nice momento for the student teacher.
This may sound strange but I think a hand chain would be great. Each student can trace their hand and cut it out of black construction paper and fasten them all together in a chain. This teacher can remember her first group of kids by the hand prints and know that she had a "hand" in their education. They can write their names on the black paper with whiteout or a metallic silver sharpie. It will be quick, easy, fun and something she will cherish. You are kind to think of her with a surprise.
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