A Massachusetts girl's awful experience on a school bus is at the heart of a case argued in the Supreme Court Tuesday over limits on lawsuits about sex discrimination in education.

The 5-year-old kindergarten student in Hyannis, Mass., told her parents that in 2000 a third-grade boy repeatedly made her lift her dress, pull down her underwear and spread her legs.

Local police and the school system investigated, but found insufficient evidence to bring criminal charges or definitively sort out the story, according to court records. The district refused to assign the boy to another bus or put a monitor on the bus, records show.

Upset with the school district's response, parents Lisa and Robert Fitzgerald sued the district in federal court under both Title IX, which bars sex discrimination at schools that receive federal money, and a provision of a Civil War era, anti-discrimination law that was designed to enforce the 14th Amendment's equal protection clause.

The issue for the court is whether Title IX, enacted in 1972, rules out suits under the older provision.

A federal judge ruled that the Fitzgeralds could not sue under the older law because Congress had subsequently passed Title IX. The Fitzgeralds also lost on their Title IX claims. The Boston-based 1st U.S. Circuit Court of Appeals upheld the ruling.

The justices appeared skeptical of the idea that Congress, in legislation expanding protection from discrimination, would cut back on the ability to sue for violations of constitutional rights. But they also wondered whether the Fitzgeralds ultimately would win their lawsuit.