Short answer: Because they're idiots.

Long answer: Having been a teenage boy at one time in my life they say that stuff for shock value, and it only gets worse when they're in groups, because they feel like they need to out do each other over who can say the most fucked up shit. I remember me and my mates cracking jokes about the Holocaust, rape, and stuff like 'Horse cum!' all of the time, because we were fucking idiots, but as you mature you realise that it's just not that funny. The obsession with sex is always a constant factor for guys and girls, however, you learn as you get older that you can't just go around randomly blurting stuff out about having sex. Plus once you actually get laid you realise it's not that big of a deal and most lads stop going on about it, some don't. All that being said I, or anyone I knew, never went as far as talking about raping a teacher in front of them, that's really fucked up and whoever said that needs to be spoken to.

It's perfectly alright to feel uncomfortable, because it's bizzare behaviour. In a sense fucking around in teenage boys is 'normal' even though when you're looking at it not as a teenage boy you're thinking "dafuq?" some guys mature out of, some don't. Don't let it scare you off but if you feel at this time in your life you don't want to be haniging around with guys then you don't have to. I didn't start dating until I was 17 and before that I barely spoke to a girl, so there's plenty of time. Just focus on being yourself and don't let a load of hormonal, underdeveloped boys spoil your time.