well, i don't know about pre-treating with chemical products, but i do know that lice aren't as productive in an oily environment. and while children with fine, straight, caucasion hair may not be able to get away with having oils like coconut or olive left on their scalps and hair without it becoming an aesthetic issue, i think it would be smart to consider an oil leave-in treatment for your whole family since they all have textured hair. not to the point of an oil slick when you touch it, but enough to coat the scalp and strands from root to ends. and keep it that way on a daily basis until the lice is eliminated from the classroom.
also, alaffia makes a great black soap shampoo that has neem oil, tea tree, rosemary, and lavender, all of which are supposed to be natural lice retardants.
is your child's school a nit-free school? some schools surprisingly allow children to return to the classroom with nits in their hair, as long as living, moving lice aren't detected. but the eggs will eventually hatch, so i don't see the point. my friend's children's schools don't have a no-nit policy and the lice eventually became a school-wide issue for several months.
where i live, the public schools have a no-nit policy. if they see even one nit on one strand, they won't permit the child to return.
good luck. lice can be mentally taxing to deal with, especially when not all parents are being diligent in eradicating them and are still letting their kids play and share and be around others when their heads are still flaming hot zones.