They either know, or they don't know. I had lots of great oral sex in HS and this was long before those boys had access to anything stronger than Playboy or Hustler - and back then even THOSE were like looking at Cosmo now. They just KNEW!
If they DON'T know, they just need to be taught. My ex-husband didn't know what a clitoris was until I taught him
Thankfully I'm at the age where the guys I date have been taught by others so there's no time wasted. One guy back in my late 20's is who taught ME where the G-spot was. I STILL hook up with him when I can!!