The No Nit policy, which is practiced in some schools but not others, is about excluding children from school if they're found to have nits, and not readmitting them until they are declared nit-free. There are many problems and much confusion associated with this policy, and advocates for each side make a compelling case.
Children with nits may not be in school (the "pro" side):
When a school nurse, parent, professional checker or pediatrician does a quick, dry check, and proclaims a child to have just live bugs but no nits, or just nits but no bugs, it's so very frequently not the true case. Similarly, when they proclaim a child to have just a few nits, it often ends up being dozens or way more. Live bugs are quick and adaptive -- they run and hide when someone's searching for them. And nits are so tiny that these checks usually don't find all of them. So although the spirit of No Nits is great, the checkers simply can't know for sure when all the nits are gone anyway, or even if there are live bugs. I've seen this happen over and over.
Children with nits should be allowed to remain in school (the "con" side):
It is estimated that 12-24 million school days, lice-related, are lost annually by children in the United States. The annual economic loss owing to missed workdays by parents who have to stay home with their children is somewhere around $4-8 billion. Of course this is atrocious, and the immediate reaction is to say that we need to be less conservative and cautious when dealing with the decision about who does and doesn't get to remain in school. However, this likely perpetuates infestation within a group.
The plusses and minuses of the No Nit policy could be debated endlessly. In its intention, it promotes correct awareness, because where there are nits there are likely to be bugs, or nits that will hatch at any moment. However, on paper, it's not possible to prove that the policy is reducing the estimated 6 - 12 million cases in the U.S. each year. The bottom line is that professional treatment is the only sure way to minimze the number of lost days. That's the only way you'll know for sure whether or not your kids are ready to be in school, whether or not you're helping to eradicate an infestation rather than unwittingly perpetuate it. Where there's even one nit, there may be more that will go undetected, they may have the opportunity to hatch. Where the was at least one bug (to lay the nits), it could still be there along with others. And it's beyond the scope of what any dry check can determine.