WHEN IS A contract not a contract? Is it when either party to it doesn't like it? If you go to school for an answer, that's what you're likely to hear--because that's what teachers on the one hand and elected officials on the other are arguing in these tight times. The trouble is that this redefinition is dangerous in two respects: it could cost taxpayers more than they literally bargained for and/or it could make all bargaining meaningless.

The idea of "renegotiating" a contract is nothing new; agents for professional athletes have managed to parlay this game into millions of extra dollars for their clients. Team owners, caught in a pattern set by colleague chumps, are always tearing up agreements, agreeing to higher salaries and charging it to fans at the gate. But when it comes to teachers or other public employees, taxpayers pick up the tab.

Right now, there are two versions of this game between teachers and boards in this region. In the District, the apparent new president of the Washington Teachers Union, James Ricks, is making noises about seeking to renegotiate the current contract. And in Montgomery County, several members of the county council are saying they may press the county school board to seek renegotiation of a two- year contract with the teachers that was approved last year.

Teachers may find Mr. Ricks's proposal appealing, of course, though they might well consider that the process could cost them in other contractual ways. But city taxpayers, who would foot the bill for any salary increases, should sense the dangers immediately. Similarly, an effort by the Montgomery board to reopen bargaining could make matters worse.

The safeguard in each case, presumably, will be the refusal of one side to chuck the contract and start over. But in either instance, taxpayers should be wary of attempts to change the rules--and the cost--after the contract has been negotiated and signed.