Quoting Lizplustwo:" Woah. Take a deep breath. If you did all that and didn't find anything...most likely it's gone. It's just lice, it happens. Doesn't mean you or your daughter are dirty or anything. "
Totally. I had a friend come spend the night with me once and turned out she had lice, which my mom found. We were like 9. She cried and was so embarrassed, and she kept saying she didn't want me to think she was gross. My mom told her not to feel embarrassed because lice like to live in clean hair, so it just meant she was clean
I always remembered that lol
But yeah OP if you didn't find anything after doing all that, it sounds like maybe they caught it before she was actually infected. Could be that a bug jumped off someone else, laid a few eggs which the got at the school, and then jumped off.