My first thought watching this was "awww this just shows how much they love him"
But that thought got my wheels turning...
Cuz I def remember male teachers in high school that the girls would lust after cuz the teacher was a "cutie" and "so cool"
It's one thing for a child to crush on you from a distance but it's another for them to be given a green light to touch you in any way
At that point you become a participant in whatever fantasy they have going on in their minds - whether you know it or not.
He may not be aware of it but at least one or more of those girls could have a crush on him
And considering that , this isn't very acceptable . Even if he himself has the purest intentions
Ultimately this is unprofessional and he probably shouldn't do it again