I had the same problem when my daughter started school. You go through the agony of combing, treating and checking the whole family for the problem to be re-ignited a few weeks later.
Definately send a letter to her class teacher, the school will then send a letter to all the parents of her class informing them to treat their children for lice. There is no shame attached, nobody knows where they started.
If your daughter has long hair, make sure it is tied up or plaited, so that the risk of contamination is reduced. If you go to the chemist and ask for a preventative spray you can also reduce the risk of catching them again.
Teatree oil is supposed to repel the little blighters, so if you buy shampoo that contains this or just comb it through her hair you should stand a chance.
However, not all parents are as diligent and unfortunately I had problems right up until my daughter was a teenager with reoccuring nits.
Good luck.
Deb