WHAT CAN A public school teacher expect for doing an exceptionally good job in the classroom? In most large urban school systems, the sole reward may be nothing more than a bundle of new grief: maybe a more crowded classroom the next time, perhaps a compliment from the principal and even some condolences from parents when the RIF notice arrives. By and large, there is little formal occasion to bestow real rewards, like money, even though educators and now, in general terms, the Reagan administration have at various times talked about merit compensation.

In Washington's Ward 2, however, people are doing something special to honor their top teachers. R. David Hall, the ward's school board representative, has worked with the schools and businesses in his neighborhood to establish small cash bonuses-- $150--for teachers selected in each school by principals, parent-teacher associations, teachers and student bodies. This season, 13 teachers were picked under a weighted voting system that seems to have won wide acceptance. The awards were made at a ceremony at Shaw Junior High School. Businesses contributed by purchasing advertisements in the program.

Mr. Hall is now seeking to expand the idea and to put together an evaluation procedure for measuring classroom progress and rewarding teachers who make it happen. Is it a good idea? Most people think so, but not Washington Teachers Union president William H. Simons, who says, "The idea sounds good, but it's not feasible; it's impractical and it won't work. . . . Instead of promoting better teachers, it will end up fostering disharmony."

It doesn't matter, apparently, that there are prizes in this world for employees who excel in other fields, for students who win contests, and even for union members who perform exceptional service. Would rank-and-file teachers really rather reject all cash rewards? Is the union negotiating minimum salaries at each level, or is every minimum supposed to be a maximum for every teacher regardless of effort and performance?

Mr. Hall and his constituents are on to something good, and most teachers, parents and administrators know it.