Use a louse comb and conditioner on everybody in the family(you may find a magnifying glass useful).Step one apply conditioner this clogs their breathing apparatus and causes them to remain totally still.
Step two comb,starting from the back of the head from the roots up,every part of the head paying attention to the back of the head and around the ears,have some newspapers spread on the floor.Comb through until you can't find any nits or lice burn the newspaper and it's contents.
Repeat 4 times at 3 daily intervals at the end of this time check thoroughly if something is found repeat the whole process from step one.
This method works if followed strictly also tell the school so that they can alert other parents and tell them to use the exact same treatment advise them that unless it is universally adopted and strictly adhered to then you will have continued outbreaks.We had similar problems my wife is a teacher and she caught them from one of the kids in her class and after we got that under control our kids got them we found the above method to be the most effective the pesticide shampoos never get them all and one intact egg or adult is all it takes.
The schools in Denmark hand out information leaflet to all the kids in the school to take home to their parents showing them step by step with pictures the above method.