As the mother of four daughters, this concerns me.
My daughters have brought home lice from school.
It is so annoying.
With boys, all you need to do is give a buzz cut. Problem solved.
With girls it's much more complicated.
I have found the "trick" is to cut their hair, wash with the lice shampoo, then comb with a nit comb minimum three times a day.
For girls who are used to having long hair, a hair cut is somewhat traumatic.
The first time they had lice, I was going out of my mind, and we were trying to sell the house. So finally, I took the clippers and cut a good part of their hair right to the scalp (using a number 9). It wasn't completely bald, but really short. (The top part of their heads still had a good deal of hair).
Then when the lice was gone I took them to the hair dresser to get my hair-cutting mistakes "fixed". I'm no hair stylist. But the lice had to go, especially with the sale of the house riding on this.
I had subsequent episodes, and with experience, I learned to deal with the "crisis" (with four daughters and a myself, it is something of a crisis).
The nit comb is key. People try to solve the problem with just the shampoo. The shampoo is just the first step. You have to comb until all those suckers and their eggs are gone.