I don't know the situation at your school, but I assume that if they
bought 700 licenses, they have 700 computers on which to install
an OS. Unless they are all identical, it would be a logistical nightmare
to install Gentoo on 700 boxes. And how long would that take?
How much bandwidth would it take? There are many more factors,
but these are just two which would preclude your school choosing to
install Gentoo. For that situation a binary distribution might be better,
depending upon the needs and use of the OS.
Read this thread titled
The woes of Gentoo -- especially posts #12 & 22
by bughead1. He's a sysadmin with a lot of experience.
Finally, you should submit that letter to your English teacher first. If
English is your first language, you really need to spend a little of that
computer time studying the language. Your other teacher may not read
your letter with such poor grammar.