Shut up

How long have people been saying the words ''shut up''? There was once a movie where some black guy went back in time to the medieval times and people were saying shut up, did people really say shut up back then?

No. The movie was using an anachronism to be funny. ("Anachronism" means something that is out of its proper part of history.)

I did find these references in the Oxford English Dictionary to "shut up!" in the usual sense being used in the mid 1800s:

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m. intr. (colloq. or slang.) To shut one's mouth, to stop talking. (Cf. j.) Often in imperative.

1840 Picayune (New Orleans) 10 Oct. 2/4 The Dutch~man got a hint to ‘shut up’ from one of the officers. 1853 ‘C. BEDE’ Verdant Green I. viii, Order! or-der! shut up, Bouncer! 1858 TROLLOPE Dr. Thorne v, On this occasion he seemed to be at some loss for words: he shut up, as the slang phrase goes. 1905 E. GLYN Viciss. Evangeline 134 He nearly had a fit, and shut up at once.
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And there were references to "shut up" meaning "to close a topic of discussion" in the 1600s. But it didn't seem to be quite the same -- less rude, not generally an order to someone else.

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j. Of a person: To end one's course of action (obs.); to bring one's remarks to a close. Now rare. (Cf. m.)

1628 BP. HALL Contempl. xx. Joash & Elisha 21 The Joash of Judah having been preserved..by Jehoiada the priest..shuts up in the unkinde murther of his sonne. 1657 J. WATTS Scribe, Pharisee, etc. I. 72 And now (to shut up) I will give you a brief recapitulation. 1700 R. CROMWELL Let. in Eng. Hist. Rev. (1898) XIII. 121, I fear how farre my penn hath runn; it is but reasonable to shut up. 1868 THIRLWALL Lett. (1881) II. 175, I must now shut up.
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There are times when parents tell their children and teachers tell their students to shut up. Is it OK or wrong?

In all the schools I went to, it was normal for male teachers to yell "shut up" at their pupils when the class got out of control. The teacher's exclamation was usually coupled with a banging on the table and a fierce expression. My 8th-grade maths teacher and my P.E. teachers were the worst "shut up" yellers.

Aye, I would say that the society in which I live believes that it is more than acceptable to say "shut up" to noisy or nagging children.

Aren't teachers not allowed to tell students to shut up, isn't it wrong for them to say it?

Aren't teachers supposed to say Be Quiet or hush

"Shut up" is mildly rude in America. Parents try to get their little children to use other words, and teachers usually don't say shut up to little children. It's disrespectful, and if you said it to your boss, or if you were a kid and said it to your parent or teacher, you would probably get in trouble for being disrespectful. Kids say it to each other all the time, as do adult friends, but usually in a joking way.

There is also a trend these days to say "Shut up!!" as a substitute for "No way!" or "I can't believe it!" or "Wow!" This is usually used by teenagers or young adults, among each other.

Well, after Year 5 (Grade 5, primary 5, whatever), male teachers don't seem to care anymore about the rudeness of "shut up".

It probably depends on where you're from, too. I just don't think any of my teachers would have said that to us unless we were really being very loud and obnoxious.

It would be considered very rude to say "Shut up" at any of the schools I have attended. At the current public high school I go to, (which is the best one in California supposedly) teachers often curse in front of students (s-word, c-word, b-word). It is kind of shocking to me!

What school do you go to, Californian?

is it OK for policemen to tell people to shut up?

It's not really polite, but they probably would, especially to a suspect.

I'm American and I say shut-up all the damn time no one is going to tell me I can't.