THE PRINCE GEORGE'S County school system got some impressive and well-deserved bouquets the other day when President Reagan stopped by Suitland High School to praise the school and the "magnet" plan of which it is a part. Standing under a banner reading "Great Things Are Happening," the president lauded school superintendent John A. Murphy for rejuvenating the system and -- most important from the president's point of view -- for offering P.G. parents "choice." Parental choice in schooling -- magnets, vouchers, tuition tax credits -- is an important part of the Reagan administration's educational philosophy, and education is expected to figure prominently in tonight's State of the Union address.

Programs such as the one in Prince George's have been so successful, the president told the audience at Suitland, that he wants to increase federal funding for magnets. It's not that much money -- another $18 million tacked onto the current $75 million, which is distributed in up-to-$4-million chunks. But the president's words are a tremendously strong spotlight on the concept, the biggest publicity boost yet for a specific education reform strategy.

Do magnets deserve the primacy? Are they the single best idea around? Not in a vacuum. It should be taken as no slight to the P.G. County system -- where great things are indeed happening -- to note that the president's emphasis on parent choice misses a considerable part of the excitement. When Dr. Murphy arrived as superintendent in 1985, scores and morale were low and several court-ordered busing plans had failed. Dr. Murphy did more than launch the comprehensive desegregation plan, which uses an array of special programs in addition to magnets -- training academies for principals, higher salaries, intensive concentrations of resources on schools that have been ruled too difficult to desegregate, Project Success programs for kids in serious trouble. He also galvanized teachers and principals with "applied anxiety," giving principals more authority -- and holding them accountable. He declared at the outset that in five years he wanted students averaging in the 75th national percentile -- that is, higher than three-fourths of the nation's schoolchildren -- on standardized tests. Not just white students, not just middle-income students -- everybody.

Three years later, scores are up to 73 percent in some age groups, and the system hums with enthusiasm and success. Is it because parents can choose where to send their kids? Partly. But only a fifth of the students are actually in magnet programs, and no matter how much they expand, by definition such selection-based programs have to leave somebody out. Just as crucial have been the programs, such as Project Success, for students who struggle, who aren't properly equipped -- yet -- to compete or to choose. County officials say they would like to expand Project Success next year, if they can find the money. Alas, for such programs the federal government has as yet offered no new funding or special spotlight.

The gap points up the still-too-narrow vision of the Reagan administration's approach. Magnets work well, and in Prince George's County they have created a powerful excitement. But without Prince George's-style attention to the rest, a magnet plan can end up serving only the strong -- and ignoring those who need federal attention and encouragement the most.