Why is it that high-schoolers react like this to swear words in a movie for class, but not when they watch it on their own time?

School is more restrained in expression than regular life is. Most kids hear their first bad word from the mouths of their own parents and eventually get used to hearing bad words from their parents.

I know, but can you appreciate the meme? Do you have any idea how immature the kids at my high school act when they hear that shit in movies?

I haven't personally witnessed it, I'm not a teacher. But I am disturbed that you see being justifiably shocked by something as "acting immature". If tomorrow terrorists blow up a skyscraper, would you attack all the survivors scared out of their minds after personally witnessing that for "acting immature"?