As contract negotiations between the Belmont Education Association (BEA) and School Committee stretch into 18 months, school leaders say the current proposals put the parties $600,000 apart this school year alone. The BEA and School Committee are currently in state-mediated contract negotiations for Unit A, the bargaining unit representing Belmont’s educators. Negotiations for Unit C — the administrative and clerical assistants and Unit D — representing the professional aides and classroom assistants — will resume once an agreement with Unit…