ST. CHARLES, Mo. — When the Missouri Supreme Court upheld a law in June allowing students from failing school districts to transfer to good ones, Harriett Gladney saw a path to a better education for her 9-year-old daughter.

But then she watched television news clips from a town hall meeting for the Francis Howell School District, the predominantly white district here that her daughter’s mostly black district, Normandy, had chosen as a transfer site. Normandy, in neighboring St. Louis County, has one of the worst disciplinary rates in the state, and Francis Howell parents angrily protested the transfer of Normandy students across the county line, some yelling that their children could be stabbed and that the district’s academic standards would slip.

“When I saw them screaming and hollering like they were crazy, I thought to myself, ‘Oh my God, this is back in Martin Luther King days,' ” said Ms. Gladney, 45. “ ‘They’re going to get the hoses out. They’re going to be beating our kids and making sure they don’t get off the school bus.’ ”

Public schools here in the St. Louis region, as in many other metropolitan areas across the country, have struggled for decades to bridge a wide achievement gap between school districts — a divide that often runs along racial and socioeconomic lines. By affirming the right to transfer students out of failing school districts, the State Supreme Court opened the doors for hundreds of families to cross the lines and move their children into better schools.