The teacher could remind the student to open their mouth a bit and let the sound out. However, it may not be possible for the student to do it immediately. If the jaw is too tight and there is unnecessary muscular tension, then freedom of the mouth parts – the articulators – might take some time.
Most often, this condition would indicate that there is a problem at the sound source, and so the registration would have to be worked on so that the mouth parts could be freed of tension. Opening of the mouth would then be possible.
With exercises consisting of pitch, volume, and vowel that reflexively stimulate activity in either the arytenoid system or the crico-thyroid system, then incorrect tension is released. Included in these exercises can be certain staccato forms and consonant repetition that assist the freedom of registration and articulation.
When correct vocal cord movement is restored, rigidity releases, felt everywhere as freedom to move. The mouth can open naturally, given the pitch, vowel and volume.
Functional freedom is always the goal!