Just completed another project where 40 students used (old) Google Sites (because they are Apps Scriptable) as Portfolio Sites.
Each student gets a copy of a template site. After they have worked on it, and the deadline has arrived, they are given a COPY of their site (and made EDITOR), and they are removed from their original site.
If the student wants to invite a personal Gmail account to this new site, and then that personal Gmail account makes a Copy of it, it is removed completely from the york domain - meaning when the student leaves, it won't be deleted. They can keep it forever.
They THEN can, if they choose, upgrade their site to New Sites and make it look a bit fancier.
If you want to do this, make your Template Site in Classic Sites, then...
File > Make a copy , fill in your students
edit the code Tools > Script Editor where it says CHANGE_THIS
Here are the instructions for students.