Ehm, I think most schools have agreements with ASCAP and BMI that cover this scenario... perhaps through something like this: http://www.ascap.com/licensing/pdfs/imla_rate_schedule.pdf
Certainly I haven't heard of them talking about releasing a recording on Glee, although I missed a couple episodes...
EDIT: I see that the article discusses in particular the videos that were created which would constitute "recordings". On the mashup issue, presumably they aren't actually recording mashups but writing them to be performed live - these aren't lip syncing groups, and most glee clubs probably don't make videos either so it seems fairly moot.
I thought it was very common for performing artists to record their own work and even to share it with other people on the internet. Youtube has many many such recordings. ASCAP licenses will not cover these activities.
I guess there's no problem as long as performing artists never record or post such recordings online.
Half of them are carrying video recorders in their pockets. Sure, it's very low-fi stuff, but you're remarkably out of touch if you think that teenagers don't record themselves performing and upload it to YouTube and Facebook.