My daughter has been to child care and to 2 different schools. All have had occasional outbreaks of headlice (the previous, expensive private school had the most! One kid had lice in his EYELASHES!!). She has yet to have headlice, but it will come, it will come. I bought the chemicals, but they sit in the bathroom cupboard waiting.
This site is very helpful - recommends school-wide nitbusting days. Busts a bunch of myths , too.
It does point out that the efficacy of tea-tree oil as an anti-nit treatment is unproven. Tea-tree oil has been shown to be an antimicrobial, not an insecticide (did some work on it myself, many a long year ago 
). Also tea-tree oil can provoke an irritation reaction in sensitive (think young scalp) skin.
One thing that has been shown to work is thick, gluggy conditioner and a comb.
And you get nice, shiny hair too!