I know it's against subreddit rules to just drop a link and drop out, and I'm hardly qualified to say anything more on the subject, but nobody has answered your question yet and I remembered an NPR article that answers your question exactly
[Teachers are mostly women because] as school reformers began to realize in the 1820s that schooling should be compulsory — that parents should be forced to send their kids to school, and public education should be universal — they had to come up with a way to do this basically in an affordable manner, because raising taxes was just about as unpopular back then as it is now. So what we see is this alliance between politicians and education reformers in the early 19th century to redefine teaching as a female profession. They do this in a couple ways: First, they argue that women are more moral in a Christian sense than men. They depict men as alcoholic, intemperate, lash-wielding, horrible teachers who are abusive to children. They make this argument that women can do a better job because they’re more naturally suited to spend time with kids, on a biological level. Then they are also quite explicit about the fact that [they] can pay women about 50 percent as much — and this is going to be a great thing for the taxpayer.