ETA: They didn't close my high school during a measles outbreak in...'84, I think. I had classes where half the students were out sick, and we just kept chugging. I wonder if this is some kind of strange regional/cultural difference or something.
1) so many kdis out sick that you can't be teaching new material anyway
2) disease without a vax to prevent it.
In the case of measles, first response is to quarantine cases and then vaccinate/booster shot those who are still well, rather than shut down the school these days. I know I've heard of daycare and school policies for such cases wherein unvaccinated children will be sent home for the duration of the outbreak.