I think it originates in dance music circles, where 'old school' is classic tracks from a really really long time ago ... like the 1980's! Several compilation albums have this title, and so the usage has passed into general speech
It's not just dance music. The rise of The Darkness has lead to talk of 'old school metal'. It's not meant to be rhyming slang, it just refers to stuff that was good in the past.
Old school (written old skool) doesnt mean cool its just means its either old or it sounds old. As mentioned often used in reference to samples or sounds similar to older music of the 80's. In essence it means from a previous generation. I use it all the time. Its not new either, anybody who knew Beverly Knights music before the charts got hold of it will remember her 95 track "flavour of the old skool" and it goes back way before then.