“Tighten your embouchure” is bad advice for young clarinetists.
That goes for young saxophonists, too, and really for any young woodwind players. But young clarinetists hear it often because their pitch is flat and their tone lacks focus. “Tighten your embouchure” gets thrown around as a fix-all, except it doesn’t fix all. It doesn’t fix anything. Unless your students are actually leaking air around the mouthpiece from utter slack-jawedness. In that case, they should tighten, but only a little.
The real issue isn’t embouchure, it’s voicing. Good clarinet playing requires a high voicing. (The opposite of almost every other instrument in the beginning band.) That’s why your clarinet section is flat and tubby-sounding. Tell them to blow ice-cold air, which fixes the voicing problem. Train them to back it up with powerful breath support. Let them relax their embouchures—not tight, just airtight. And enjoy the clear, full, ringing, and in-tune sounds!
I studied clarinet in college with Caroline Hartig. She had a great way of explaining/demonstrating voicing on the clarinet.
With the mouth open, teeth slightly apart, you make a really intense and forceful “SHHH!” sound. It really puts the tongue in a great position for making a nice clarinet sound.
I have had a number beginning students whose clarinet is so loose in their mouths that it will actually move around while playing or when they are attempting to tongue. Many end up playing 1/4 to 1/2 tone flat. Firming the embouchure is exactly what they need. While I would not encourage any student to bite, sometimes greater embouchure support is exactly the fix that is needed.
I have to disagree with you on this. “Firming” or “embouchure support” is tightening, and might raise the pitch but has negative effects on tone, response, and dynamic range. The flatness problem you’re describing is a classic voicing issue. The clarinet moving around during articulation is a tongue movement problem, and/or failure to bring the upper teeth in contact with the mouthpiece beak.
Voicing is harder to teach than “tighten (or ‘firm’) your embouchure,” but well worth it for the superior results.