Touch wood, but I've had children (two girls) in nursery or school for the past 3 years and no one's had nits or lice.
A total of 3 nurseries and schools, 1 in Edinburgh and the rest here.
I've heard of it being a common occurence, though.
Have you tried a Nitty Gritty comb?
Plop cheap conditioner on their heads every other day and comb with the Nitty Gritty comb. Then rinse.
Or BizNiz for a treatment rather than Hedrin or FullMarks or the like.
Also, if the children have long hair, it's probably best to wear it up for school.