I've done it. If you can move the mouthpiece around, they're not firm enough. It should be anchored to the top teeth.

I did it less this year due to COVID. I generally demonstrate it. "Here's my sound when my mouthpiece is loose." Then I pull the mouthpiece into my embouchure and the sound immediately gets better and more centered. And then I have them do the same.

We start it with mouthpiece and barrel. They hold it in the right and and pull it in. When they assemble the clarinet, I'll have them play open G with the left hand on the barrel and right hand on the bell, again pulling it in. As we learn hand position, they're eventually pulling the clarinet in with their right thumb. The right thumb and the embouchure and the only things you really need to hold the clarinet.

You don't form a firm embouchure by biting or tightening the lip muscles. You do it simply by keeping your embouchure set and pulling the mouthpiece in. The mouthpiece is like a doorstop and your embouchure is the opening under the door. The space doesn't get smaller or tighter, the doorstop just wedges its way in.

This and the angle of the clarinet is how you add more pressure without biting or using chin and/or lip muscles.