question about school lice checks

My child's class is in the midst of a bad infestation. The school brought in a professional company to check every student in the school. We already knew we had lice and had done a dimethicone treatment and combing with the Terminator comb, found some nymphs and nits and at least one larger louse. The first schoolwide check happened one week after our treatment, and my daughter was found clear. A few days after that check we did our day ten repeat treatment and combing. I found nothing this time. Three days after THAT, they've repeated the school wide lice check and they found a nit on my daughter and sent it home taped to a card... but it's more than an inch away from the end of the hair. Does that mean it's definitely old/nonviable? Or could it have gotten pulled along the shaft by their comb today? Another student has a similar situation-- the nit they taped to the card is like two inches from the end of the strand. As a side note, are these kinds of checks at school reliable? It seems to me likely they'd miss a lot of cases since the checks are pretty quick, or find old evidence like what I'm hoping ours is...?