We had a Creative Memories (scrapbooking) consultant/mom put together a beautiful scrapbook with photos collected from present and past students. It was done beautifully and the principal was delighted!
I don't know if you have the money and/or time for this, but-
Have the kids sign and write messages in colorul pens and send the sheets to Original Works (www.originalworks.com
or 1-800-421-0020) to make quilt blocks. Then a quilter (volunteer or professional) can put the blocks together to make a very nice keepsake. You can talk to your principle's spouse or a family member to see what colors to do it in. This is a great gift for someone who has performed years of service.
You could do this same idea with a white comforter and fabric pens but it isn't as sharp. I have gotten pillow cases and quilt blocks from Original Works and they look great.
If your school is a small one, places like Playful Potter do platters, where you have the kids sign their names and then it is painted and put into the kiln. It can hold alot of names and it's a nice gift. If they don't have Playful Potters where you are then any pottery store would be able to direct you.
Don't know how big of a school you are, but if you're not too big, here's an idea for a gift from the kids. Take the class pictures from each class (that they had done in the fall) and either use the originals or have copies made, glue, use doublesided tape or photo corners and arrange them on a big sheet of posterboard folded in half to make a BIG card and then pass it through each classroom having all the kids sign their names. Your principal will treasure this for years to come (I hope?). If the pictures aren't readily available ask a parent from each class if you can borrow theirs.
We've had plaques made up for retiring teachers, with the years of service, etc. on them.
Good Luck!