Negotiations between the Rochester City School District and the Rochester Teachers Association have apparently hit an impasse.
The RTA has asked the state Public Employment Board to appoint an independent mediator to resolve the dispute.
The teachers' contract expired at the end of June.
Union president Adam Urbanski says teachers are looking for lower class sizes, along with more social workers, psychologists and counselors, and a salary increase.
A spokesperson for the RCSD says they hope both sides will return to the negotiating table.