I don't have that problem. I'm an inveterate nail biter (filthy habit, I know). On the positive side, I read that nail biting is a sign of perfectionists, and it is a symptom of being stressed about not living up to our own high standards. Although, it was probably written by a nail biter justifying his dirty habit. Lol
Galatea,
as you have stated, 'nail-biting' is a filthy-habit', not one that your students should see
or learn..it's always so sorrowful to see someone whose nails are deformed by biting them...
by the way, when I was young, I bit my 'toe-nails', but thankfully out-grew it...