How to display one to many mapping in a Grid

I am having some trouble finding the best way to display this sort of mapping.

My current grid looks something like this

However there is a 1 To Many mapping between a Username (plus the rest of the details) and School.

There is a 1 to 1 mapping between School and School Groups. School groups would generally either be by district or if a private school, then all affiliated schools.

I have had a look at the following related question but due to how the School - School Group mapping works, the suggested answers would not work here.