A schoolteacher is a person whose job is to teach children at a school. We usually mean someone in a primary school or a secondary school — the schools you go to before university. In everyday talk people mostly say teacher; schoolteacher is a little more exact, or a little more old-fashioned, when you want to mark “this person teaches at a school,” not at a university and not only in private lessons at home. A university teacher is a lecturer or a professor, not a schoolteacher, though learners often mix these up. You hear schoolteacher a bit more in American English; in Britain, teacher is the normal word unless someone is being careful.