I would expect the effect on health (in either direction) to be dwarved by stuff like: detrimental effects on understanding people (even as an adult I struggle quite a bit to communicate when both sides wear masks), especially if your teacher is masked, or if you’re learning English as a foreign language; detrimental effects on socializing (humans have various instincts for reading facial expressions which have not evolved to take masks into account); etc.
I would expect the effect on health (in either direction) to be dwarved by stuff like: detrimental effects on understanding people (even as an adult I struggle quite a bit to communicate when both sides wear masks), especially if your teacher is masked, or if you’re learning English as a foreign language; detrimental effects on socializing (humans have various instincts for reading facial expressions which have not evolved to take masks into account); etc.