Possibly, I suppose. But that wasn’t generally how it was interpreted by the class when I was at school. It was more “time wasted now has to be made up in your own time at the end”.
I’ve been a substitute teacher in the same district for over 20 years. I will either lean against the desk, looking at my watch quietly, or I’ll just say, “Oh, sit down and shut up,” also quietly, with a slight hint of exasperation. It’s worked so far. I rarely have to raise my voice. A good teacher told me very early that I should never be adding to the chaos of a disorderly room.
I agree with this. I heard that you have a short time - even seconds - to capture and keep peoples’ attention. Get straight to the point, or else you’ll lose them.
Susan
Heh. Back when I was a deputy sheriff, I nicknamed my shotgun The Minister. All I had to do was rack one into the chamber, and the individual that was the object of the exercise would reach to heaven and yell, “JESUS CHRIST!”.
“Listen up, maggots. You are not special. You are not a beautiful or unique snowflake. You’re the same decaying organic matter as everything else.” - Tyler Durden - Fight Club
AVAST MATEYS!!!
Actually, in the business world where I mostly deal with adults who are indistinguishable from children, I usually start off with “Okay…let’s get started” in a clear but not excessively loud voice. Also, if small pockets of people are still having conversations, I will continue speaking, but I will walk near one of those pockets and talk a bit lounder.
Listen up children.
You rapscallions.
Do I need to get medieval on on you.
OK you win, we can stay in for recess and work on this.
If you don’t behave I won’t give you the antidote before you go home.
-
Turning off the lights for a moment will get their attention.
-
One that I really like but never use: set the rule that when they see you raise YOUR hand, they raise theirs and face front. That is the signal that you need quiet. As others see their classmates hands go up, theirs go up and pretty quickly it’s all quiet.
Really? For saying this as a general instruction to the class? If you say it to one kid who’s kept yakking to a friend?
That SUCKS!
So, er, you can’t whip them with a belt either?
When I was a kid… 
This approach (NSFW!) would definitely work. You might not have a long career, but you’d Definitely get their attention.
In my experience, it largely depends on the school and what the pupils expect to achieve.
I’ve taught computing to graduates who needed to pass the course to get a well-paid job.
Easy and rewarding for all of us - they really wanted to learn.
I’ve taught chess and roleplaying to kids who volunteered to come in on weekends for my class.
Easy and rewarding for all of us - they really wanted to learn.
I’ve taught computing in technical college to teenagers who had changed from school because they weren’t academic.
The approach needed some thought, but once they saw the practical applications we got on fine.
I’ve taught computing and substituted for colleagues in Maths, History, Music and English. As this was a private school, the pupils stand up when you enter the room. 
My approach with all new classes is to introduce myself - “Good morning. My name is Mr. Glee.” - and look confident.
But I appreciate there will be huge potential problems in schools with large class sizes where the senior staff don’t back you up and the kids are just waiting to leave school.
I guess my equivalent would be putting on an Aussie accent and announcing
G’DAY MATES! CALL THAT A KNIFE? NA - THIS IS A KNIFE! 
Where do you work? 
I’ve told kids to be quiet in front of Governers, Headmasters, Heads of Department, School Inspectors, colleagues and parents.
You stand at attention. Say:
“Achtung!..Silencio!”
There’s nothing wrong with telling them to be quiet. You just can’t say “Shut up”.
or, “OK, people…”
If it needs to be an attention grabber, just shout ‘HEY!’ followed by a declarative instructional statement. The ‘hey’ has to be loud, fast, and have shock value…like the verbal equivolent of slamming a dictionary down on a desk.
“CONSTANT VIGILANCE!!!”
Or perhaps “SILENCE, INFIDELS!”
Point out the window and scream " Oh my God, a flying saucer"
That’ll shut 'em up
I’ve barked “Shut up!” at kids whenever I needed to (admittedly not often).
I have 27 years of teaching under my belt and have received glowing reports from all my employers.
What are you referring to?
Oh, geez! You’ve just triggered some serious childhood trauma. I have no idea what the joke was, but in the fifth grade, over the lunch hour, they gtot my classmates father, an army sergeant to “supervise” the class over lunch. He stood at the front in full uniform with his cane… and I don’t think he blinked once. We ate in total silence. When my classmate, Sean, was diging around in his schoolbag to find his thermos, Rob’s dad slammed his cane on Sean’s desk, which made Sean scream.
Without doubt, the scariest lunch in the history of my childhood. I don’t think I actually breathed until the bell that let us out for recess.
The worst problems I ran into were in middle school, where the kids were just coming into their own as being young punks with hormones and no idea whatsoever what to do with them. There were some good kids, but there were also bad kids who didn’t give a shit about what you were teaching and knew that you couldn’t really do anything to them. You can’t touch them, the principal doesn’t want to deal with them, and they ditch detentions because their parents don’t care.
I wasn’t the best disciplinarian in the world, and I made some mistakes about teaching in general, but there are some kids who don’t really want to be reached. It’s sad, and I wish I had better answers.