It sounds like you are looking for something known as 'multi tenancy'. There isn't a straightforward answer to your question and it depends on exactly what your requirements are (e.g. does each school need the same plugins? do you need local admins for each school? what exactly can those local admins edit?)
Here is a document that describes some possible considerations / approaches: http://www.synergy-learning.com/blog/wp-content/uploads/2012/05/Multi-tenancy-in-Moodle.pdf
(I'm sure you can find out more info by Googling 'Multi tenancy Moodle').
For future reference - this forum is for discussion Moodle development issues, there is a 'general help' forum for questions about how to use / configure Moodle: https://moodle.org/mod/forum/view.php?id=50