You can manage your whole team through the School page: from here you will be able to add Ambassadors, Class Admins, Community Reps and School Staff*. For more detailed help on assigning roles click here, or for more information on the functionality of all the roles, please click here.
NB: A parent can be a Class Admin for multiple classes. This can be especially useful in secondary schools where one person might cover all classes within the same year.
To change someone's Admin status after they have been appointed, stay on the School page and simply click on the parent you want to change. This brings up their details and you can click the cogwheel icon to change their Admin Status.