“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!