Because I think this sounds really lame, especially in front of a class of kids; as if you were asking their permission to speak (OK, guys, will you let me talk now, if you don’t mind?). And it’s really illogical when things aren’t “OK” and I need to project some kind of “I’m really serious” vibe. And “Hey, guys” doesn’t work either; it is a bit stronger but still sounds kind of lame. (FYI I’m a substitute teacher).
Can you blow a whistle?
It’s enough out of the norm that you will probably get a few seconds of shocked silence, then you can jump in with whatever you need to say.
There is always this approach. 
I recommend putting on a British Accent and announcing
RIGHT! STOP THAT! IT’S SILLY!
I use that at work sometimes when the other guys are talking about Anime or something equally unproductive instead of doing their jobs. It’s surprisingly effective. 
That was hilarious. Unfortunately, you can get fired for saying “Shut up”. :rolleyes:
I had a professor in college who would simply to start talking in a slightly quieter than normal voice. The first few times he did it, it took a few minutes for everyone to catch on. After that, it produced almost immediate silence. It helps to answer the inevitable “Can you repeat what you just said?” questions with “No.”
I had a wonderful history teacher when I was a lad (Ms Gibson of Terrigal, if you’re doing a search, I say Hello)…
She would start every lesson the same way, regardless of whether the class was behaving as angels, demons, or any form of smelly teenageness in between…
No “good morning”, no “hi”, no “Right. Shut up!” In fact, no nothing. She would just walk in, drop her papers on her desk, and say, “In 1815, when the explorers Blaxland, Wentworth, and Lawson found a passage across the Great Dividing Range to the inland…” and the lesson was on.
It worked excellently. Every time.
Ah, subbing…the memories! I’ve been teaching now for over twenty years, but I started out subbing, which really gives you the discipline tools you need, IMHO. I was also a professional storyteller at that time, so sometime early in the day, I would randomly start telling a “jump” story (a story that is told very quietly, and at the end, you yell and grab someone, usually permanently scarring their childhood). They would love it, and then I would say, “If you’re quiet and work well, I’ll tell some more stories.” I always had them in the palm of my hand from then on. Believe me, you’ve never felt a rush of power unless you’ve made a six-foot sophomore knock over his desk in terror. So bring something you enjoy into the experience. Tell tales of your childhood. They love it.
This is what I was going to suggest. If there are a couple of unruly ones after this, then the heads start to roll.
I’m guessing a loud “TEN-HUT” isn’t going to work?
“Hey, y’all”
If the situation gets really dire:
“Dudes!”
What? It works for me. 
I find working the action on a pump action shotgun gets people to be quiet.
I like the idea of just starting right in on the lesson. Some other ideas -
A gavel.
Some version of “all eyes to the front.”
“Listen up.”
If it is mainly one small group that is not paying attention, go stand right by them when you address the rest of the class.
You brave soul you, substitute teacher …
“Everyone . . .”
"Attention, please . . . "
“Dudes!” is pretty good and might actually work!!
“Listen up, People!” or “People…Listen up!” is what I remember coaches and teachers saying in class alot when I was in school.
[Klingon]
YOU WILL LISTEN!
[/Klingon]
It depends on the age, size and general behaviour levels of the group, and on how noisy/distracted they are. But in general, I find that ‘look this way, please’ can work as anything from a quiet request to a severe instruction. Body language is essential in all this, and can give a clear indication as to whether or not things are OK before you say anything else.
Using anything other than your voice should not be necessary. Definitely not a whistle, unless you’re in the middle of a sports field! I try to avoid even clapping hands to get attention, except for making a break from general chatter or rowdiness before starting, or in groups where I’m able to work with a more informal athmosphere. These are also the ones who I can tell to “shut up and let me talk” 
How about “the sooner we start, the sooner we finish”? Several of my teachers used to use that one and it worked well, especially in the last period of the day.
A tricky one to use effectively. Particularly as it implies that they shouldn’t be interested in anything you’re doing.
You could try “m’kay”, like Mr. Mackey.
I don’t think it works for him, though.