Most elementary teachers post photos of their students regularly. Over the years there have been many services that help create slideshows. If a school is a GAFE school, and teachers are expected to not use any outside services to share photos, options for slideshows are very limited. Picasa has been a favorite at my school, and currently Google Photos does not offer a similar option to create slideshows. It is possible to make movies from the Photos app, which I have previously documented.
Below is my explanation of how to make a slideshow from your Google Photos: