That's a dumb question! ;-)
They learn it in sex education classes that begin in kindergarten!
Oral sex is safe sex! (except for all those HPV mouth lesions that are now showing up)
Well, I didn’t learn about oral sex until I was an adult. I knew about the “facts of life” even before puberty, but, knowing details about intercourse or other sex acts? No.
Guys in school may have bragged about what they did with this or that girl, but, the subject of oral sex specifically just never came up back in the day.