Create a State on the Repeating Group of “selected school”.
When you click a row, use the “set state action” and set the “selected school” state to the row that you just clicked. Eg it might be “Parent Groups School” or “Current Cells School”.
Then create a “condition” on your checkbox so that when “Current cells school” = Repeating Groups “Selected school”, it’s checked/blue colour.
Then you can do whatever you want in subsequent steps… eg if you want to save that school to a List somewhere on the user, after you click “update”, create an action that adds the Repeating Groups “selected school” to it.
You could also achieve multi select the same way. Change your state on the RG to a list… then when you click another row append it to the list.