Lice sucks. This is something I know you know because, hey, guess what? It turns out your kid had it TWO weeks prior to the fourth-grade outbreak that struck every girl in the class (including my kid). I found this out thanks to another parent (whose daughter ALSO had lice). Not from you.
Look, we all know lice is gross. Lice is embarrassing. It took us $750 out of pocket and a lot of stress and tears to make sure that everyone else in our family was lice-free and to rid my daughter of every single nit and bug. One thing they told me: Her case was two weeks old. No one else in our family had contracted it, which implies that it was very new, which coincides to right around the time that your child had her super top secret case.
Lice is no fun. But it's especially no fun when we aren't told that our children have been exposed. Suck it up. Do what the rest of us did and tell the teacher.