Hi William,
You can show images from all galleries by doing a Recent Gallery like this: [photosmash gallery_type=recent images=4000]
You can set whatever layout you want. It will pull in all images from all galleries (assuming there aren't more than 4000 all total, you can adjust that number as needed) sorted by most recent.
At this time, it doesn't allow you to specify multiple galleries (it's either all or one), but you could tag images the images with school's name and show a tag gallery that would show all the schools images.
[photosmash gallery_type=tags tags='my school']
Unfortunately, there isn't a way to apply a tag to all images in a gallery :-(
Cheers,
Byron