You simply have to be a School that has some form of connection to the Internet. The simplest form is a single person with an email address - but it could go all the way to every pupil and staff member have their personal email address and access to the WWW in every classroom.
One definition of a school is:
(taken from the Manager of Uninet, Mike Lawrie's definition in http://www2.frd.ac.za/uninet/docs/schoolscond.html)
No. Probably not. There are schools joining the Internet all the time. They are only present in the list if they add themselves and keep their information updated.
Just as schools are only present in the list if they add themselves - if they do not keep their information updated we have no control over it.