To re-educate someone is to teach them again, so they think or behave in a new way. The re- means again: they already learned one way, and now someone is trying to replace it. It can sound quite gentle. A hospital may re-educate patients about salt; a company may re-educate staff to use a new computer system. Though actually, it can also sound hard and political. In news and history, re-educate often means forcing people to drop old beliefs and take the government’s beliefs. That use is cold, not kind. In ordinary school talk, teachers more often say teach again, train again, or help someone change a habit, because re-educate can make a listener think of that darker meaning. You re-educate a person, not a school subject. British spelling often keeps the hyphen before a vowel (re-educate); American spelling often writes reeducate.