but how would you get permission ?
Ask the copyright holder. In the USA, this is easy/trivial to
do for audio recordings (CDs, cassettes, etc.) with what is
called a "compulsory mechanical license". The fees are
set by law at a few cents per song per copy. There is even
a website where you can go
www.harryfox.com and apply
for a license online.
But for film/video, a different kind of license is required,
a "synchronization license". Alas, in the USA, these are
negotiated between the owner and the film/video producer.
There is no compulsory license, and no set fees. Because
of the nuisance factor, most music licencees won't even
talk to you unless you are producing a bigtime movie with
license fees into thousands (or tens of thousands) of dollars
per song. In other countries, "sync licenses" are handled
more like "mechanical licenses" are done here in the USA.
OTOH, if you have some personal connection to the artist
and/or the songwriter (or their agents), you might get lucky.
You could just try writing the copyright holder a nice request
on school letterhead, you might hit the lottery? But don't get
your hopes up.
would they persue something like this against a school for
1 song even given that the song is not audible all the time
as there is talking etc ?
All it takes is one person reporting the violation. In the
immortal words of Dirty Harry: "Do you feel lucky?"
USA law requires them to pursue even "minor violations"
in order to show "due dilligence" in protection of their
intellectual assets. If they just let it go, the next person
could come along and say that they don't care and have
let the song slip into public domain.