As a teacher, nits habitually come into class every autumn and winter (hat season!).
They aren't life threatening or dangerous to humans, but they are hard bloody hard to get rid of and a bit of a pain.
A letter is sent home and parents are reminded to be vigilant. Sadly, many people seem to think a treatment is enough to get rid of them, and don't change bedding, comb or get rid of soft toys. And so... the cycle of head lice begins.
Also, as a PP said, not all children itch or scratch. That is caused by an allergic reaction to the head lice biting the scalp, and varies from person to person.
There is no point in blaming people but I do always have a quiet word in the parents ears when I see them. And when I catch them (which happens!) or my DC, I always let the people around us know, as I refuse to buy into this secrecy as if it is a source of shame.
Buy nitty gritty combs, and get checking!
In another life, I would love to be a bit nurse or open a head lice clinic. I love squishing the little bastards and hearing them crack between my nails!