R.J.
Fingerprint collage?
Especially if you know anyone who does digital imaging...you can take a thumbprint from every child, scan it, blow it up to about 2 inches, recolor it, lay it on a background color square. Then get Kinko's to print it out as a poster, and get it framed. This would work for any number (1-1000)...just changes the size of the poster you need.
I would talk with Kinkos (excuse me, FedExOffice). Not only are there education/teacher discounts...but you may be able to get it donated. It might not really matter though, because it wouldn't cost that much...I THINK the pricing on their big color printer (that prints off of a roll that's 4 or 5 feet wide) is $7 per square foot. Say a 2x3 poster. You could have 600 2inch squares with thumb prints. My guess is that you only have around 100 kids max (and maybe as few as 20) so you're looking at a cost of $2-10. Which would fit in a standard frame & matte.
http://www.dna11.com/gallery_finger_prints.asp
This website has something similar on a much grander scale