depending on how much work your going to put into it and when your going to give it. here are a couple of things my kids have done for teachers in the past. room moms did the coordinating of these things.
hand print wreath.
have each kids handprint traced and let the child sign there names on it. also attach and photo. get these sheets laminated and then cut out the handprints. put them together to form a wreath and instead of a bow you make a big 2013 (my kids did this back in 98 and 99 lol) you use green a light green paper. that way she has something that can be used at the holiday and she will see the kids faces.
another teacher when my kids were in about 4th grade had the kids each dip there hands into paint and she had an american flag made. so some kids white paint some blue and some red. that went up several times each year. same concept with the names of the class. no photo's on that one.
a photo collage is great but a teacher probably won't keep that forever they just don't have enough space but something they can use as a decoration might be used.