We have a picture frames that show all 12 school pictures but the photos need to be a weird size.
Use this template to print the school pictures at the exact right size on a single 4x6 photo at Walmart or wherever.
Print as a PDF, then convert the PDF to a JPEG. If you are doing this on a Linux PC you can use this to do the conversion:
pdftoppm -jpeg -jpegopt quality=100 -r 300 ~/Downloads/School\ Pictures.pdf ~/Downloads/school-pictures