Using Google Photos on iPad is very similar to using it on a laptop, but... not the same. Hence this guide:
Create the album and copy a share link as usual, then share that link with the class, I would use a Seesaw announcement for this.
Alternatively... (the app can be buggy)
Open up one image, ... Choose Save to Device, but you need to do one image at a time...
See the carousel below for these steps illustrated!
When you share an album with your class, instead of selecting and downloading, they can choose to add the images to their own Google Photos collection, and then use those photos to create a collage, or an animation, or perhaps their own album.
The cloud icon is the one to add to their own Google Photos library, instead of downloading.
The Google Photos app, allows all sorts of options to use selected images for...