One user account can have different roles and access to many schools.
User Access
One user account can have access to different schools.
From Users-> Edit user -> select the schools to give the user access on.
User Permissions
Some users have several roles in addition to the mail role they have.
Permissions-> Select the extra role from the Role list-> From the Schools List, select the school you want to add the role to, then click save.
Note: the school administrator who assigns the roles and permissions to the users, should have access to the schools to which the roles/ permissions are to be added.