In each centre you have to set the user up using a different login name, but the same email address for each account.
The user will then need to log in using the login name and the password set by the admin.
This will then ask the user to reset the password for the account, each account must be set up with the same password.
Once this is done, the user will be able to use the email and the new password they have set to login.
When they log in, they will be given a selection screen, which will allow them to select the school they wish to log in to. If a school is not listed in the dropdown, this normally means the password has been set differently for that account and will need to be reset.