A whistle is a great tool. Another important tool to getting a talkative or distracted groups to focus is what I call “call and response”

It can be a verbal response:

“Eyes on who?”

“Eyes on you!”

Or a physical:

“If you can hear me clap once”

“If you can hear me put your hands on your head”

But something that gets kids to respond is going to refocus the group faster then yelling at them or shh-ing them.

It can also become part of your culture, the trained call and responses.

You: “abc juijitsu”

Class: “Rocks!”

If you want attention, get them to respond.