A legal challenge to Oklahoma's voluntary school prayer law got under way this week in a federal court case that could have an impact on President Reagan's proposed constitutional amendment authorizing voluntary prayer in public schools.

Two mothers in rural Oklahoma's Little Axe Independent School District filed the suit in May 1981. They contended that distribution of Bibles, voluntary prayer and teachers' involvement in religious activities -- including asking the students to pray that the parents would drop the lawsuit -- violate constitutional guarantees of church-state separation.

Feelings about the action have run so high in this Bible Belt community that the trailer home of one of the mothers, Joann Bell, was burned last year and she was attacked at the school by a school employe. In a separate court action, a jury awarded Bell $20,000 in damages for medical expenses.

Both sets of parents said they have moved out of the school district to prevent harassment of their children.

State Rep. William Graves, who wrote the 1980 law permitting voluntary prayer in Oklahoma schools, is representing the school board in the action in U.S. District Court. He argued in preliminary papers that the school board has power to establish certain religious practices.