If you want the extension number to match the room number then you may, depending on the numbering scheme, need 4 digits just for the extension. For example, if your buildings are numbered using architectural numbers, i.e. A100, B100, etc then you'll need a digit for the section (a=1, b=2, etc). Also, you likely have multiple schools for the same level, 10 primary, 2 intermediate, etc would it make more sense for your customer to have all of the primaries be 1Xxxx? The intermediates 2Xxxx?
