Thanks for your input.
SchoolName > Accounts > Devices are for accounts assigned to a device rather then a person. For example, ipad_wifi so the ipads have a login to connect etc.
External Users do have logins, and I wanted to make sure they are split from Internal so we can make our VLE use Internal. Apparently there's no other way, plus we have a fair amount of Internal and External users, so is it good to keep them separated?
Teaching Staff vs Support Staff, we have about 100 in each, again is it not better to keep them seperated? Otherwise you'd end up with a lot of users in one OU? Also, we would create sub OU's such as English, Maths, Science in Teaching and IT Support, Admin, SEN in Support Staff.