A standardized test that all Texas high school students are required to pass to graduate is the subject of a civil rights trial in which the state is accused of discriminating against minority students.

The case, being heard in a federal court in San Antonio, alleges that minority students have failed the test and been denied diplomas in disproportionate numbers.

The outcome could have an effect on other tests Texas students are required to pass to move through lower grades, under a ban on "social promotion" that Texas Gov. George W. Bush has touted in his campaign for the Republican presidential nomination.

More than half the nation's states require students to pass standardized tests to move on to the next grade or to graduate from high school. The Texas lawsuit could herald many more legal challenges to such tests and the heavy emphasis states have placed on them.

The crux of the lawsuit, filed by the Mexican American Legal Defense and Education Fund (MALDEF), is the contention that Hispanic and African American students concentrated in underfunded school districts have not been afforded an equal opportunity to learn the material covered on the graduation exam.

"Even within school districts, we don't have the same access to important resources or instruction," said Joe Sanchez, a state policy analyst in MALDEF's San Antonio office. "The best teachers go to the better schools."

The state government maintains that the exam, which has been given since 1990, is not biased. The Texas Education Agency notes that the state has a uniform curriculum, publicly discloses the "essential knowledge and skills" tested and releases a copy of each year's exam after it is given.

"We've had a statewide uniform curriculum taught in all schools for basically 15 years. Minority students and all students have the same opportunity to learn the essential skills and pass the test," said Joey Lozano, a spokesman for the state agency.

Texas has what many education policy analysts regard as a model system for monitoring the achievement of students across demographic groups. The state judges districts and individual schools not only on how well all students do on the required tests but also on how each racial and ethnic group fares.

But students belonging to different groups do not pass the graduation exam at the same rate, although each group has improved its performance in the last five years. MALDEF says that as many as 85 percent of students who failed have been Mexican Americans or African Americans, and they have failed the three-part test in reading, writing and math at twice the rate that white students have.

The lawsuit asks U.S. District Judge Edward C. Prado to prohibit the state from using the exam or any other standardized test as a graduation requirement.

"It does not take into consideration what kind of grades you have, what kind of student you have been," Sanchez said.

This trial is not the first legal challenge to the Texas test. In 1997, the U.S. Department of Education settled a discrimination complaint filed by the Texas NAACP with a negotiated agreement that required the state to provide students the "necessary curriculum and instruction to pass the test," remedial help and multiple opportunities to take the exam, according to Arthur L. Coleman, deputy assistant secretary for civil rights. Students can take the test eight times from 10th grade until the time they are scheduled to graduate.

Similar complaints alleging discrimination based on race and national origin are pending against North Carolina and Nevada.

"As soon as states shift to standards and tests as a means of achieving reform, their role is going to be scrutinized," said John F. Jennings, director of the Center on Education Policy. "That's where some states are going to be in trouble."

In the early 1980s, federal courts upheld Florida's graduation exam, provided that other conditions were met. "You have to be teaching what is tested to all kids who are being tested," Kathy Christie, an analyst at the Education Commission of the States, said in explaining the legal precedent.