Middletown, New Jersey schoolteachers went back to work today after a labor dispute that landed a quarter of the teaching work force in jail. The teachers had staged a walk-out because they refused to continue working without a new contract (the old one had expired on June 30), and they resisted proposed increases in their health insurance premiums. Last week, Judge Clarkson S. Fisher Jr. issued a back-to-work order, and when the striking teachers failed to follow the order, he began jailing them, working his way through the alphabet day by day. By yesterday, more than 225 teachers were in jail. Today, the teachers agreed to go back to work next week, and the union and the school district will enter non-binding mediation.