I would think such practice can be done from 6 or 7 meters with the instructor staying at the bottom and observing the bubbles from slow exhale as the trainee ascends.
According to e.g. this: Can you feel a lung over-expansion injury happening? – Divers' Blogs 0.1 atm can be enough to blow a lung, so anything deeper than 1 metre is not entirely safe (though at 1m you'd have to really "pack" your lungs to blow them I expect).