RIALTO, CALIF. -- Mary Amaya was happy this May when the school sent the letter asking permission to give her son, Demond Crawford, 14, special tests. He was a bright youth, like her other children, but had been struggling in school lately. She wanted to know why.

Then she found the postscript at the bottom of the school psychologist's letter: "NOTE: Because Demond is Black {the words were underlined}, we will be unable to give him an intelligence test per Peckham decision."

She thought: What is this?

Unbeknownst to her and most other Californians, a lengthy national debate over intelligence tests in public schools had just ended in the nation's most populous state, and the anti-test forces had won.

Henceforth, no black child in California could be given a state-administered intelligence test, no matter how severe the student's academic problems. Such tests were racially and culturally biased, U.S. District Court Judge Robert F. Peckham had ruled in 1979. After losing in the 9th U.S. Circuit Court of Appeals last year, the state agreed not to give any of the 17 banned IQ (intelligence quotient) tests to blacks.

That did not seem right to Mary Amaya, who prided herself on demanding the best for her four children. She had never finished high school, but she resolved to find out why and how they could shortchange Demond this way.

In the two months since, Amaya, a 40-year-old housewife with a mantel full of softball trophies, has interrogated one of the principal lawyers on the winning side, rallied to her cause much of this little bone-dry city along the San Bernardino Freeway and acquired the active support of a member of the U.S. Commission on Civil Rights.

She also has brought sighs of exasperation from the civil-rights activists and psychologists who worked 15 years to win the intelligence-test case. The government attorneys and pro-test psychologists on the losing side are expressing delight that a black child's parent is finally seeing their point: the test was just one of many tools and did not need to be discarded.

"It's . . . an interesting twist," said state Deputy Attorney General Asher Rubin, who defended the state against a suit by the parents of "Larry P.," a San Francisco schoolchild, and five other black youngsters.

Amaya has been particularly dissatisfied with the support for the IQ test ban she has heard from E. Lewis Clark, president of the Rialto-Fontana branch of the NAACP. In a letter to Amaya, Clark acknowledged that he had not heard of the new rule until she brought it up. He said that after checking with several authorities his executive committee had concluded "the overall impact of the decision will be more beneficial than detrimental to black youth."

Amaya said the letter suggested to her "that in the last 18 to 20 years, black people haven't made any progress. That man Lewis Clark is a contractor -- he builds things -- and he's telling me he has not made any progress?"

No one has yet given Demond the tests his mother wants for him, but her protest is liable to affect both sides in an ongoing legal battle over how best to categorize and help minority children who do not do well in school. Psychologists and educators in several states are discussing the issue, and courts in Illinois and Georgia have reached conclusions contrary to Judge Peckham's. The Maryland, Virginia and District of Columbia public schools still use intelligence tests, as well as other methods, when assessing students with academic difficulties.

Amaya, who is Hispanic, met Demond's father, a black factory-worker, after she dropped out of high school two months short of graduation in 1965. They did not marry, but lived together 15 years before the relationship ended. They raised four children in the Los Angeles suburbs of El Monte and West Covina, where blacks are a small portion of the population. Amaya, now married three years to a construction worker and living in a tile and stucco tract house here, rejected a suggestion by school officials that she sidestep the test ban by declaring Demond to be Latino. "I thought that would be denying Demond's black heritage," she said.

She argues that the cultural disadvantages of growing up in black ghettos, which might distort intelligence test scores, do not apply to her children, but opponents of the tests strongly disagree.

"It is an instrument that has consistently been used for discriminatory purposes," said Armando M. Menocal, a San Francisco attorney who represented Larry P. When he heard of Amaya's complaint, Menocal wrote and invited her to call him collect to discuss it. They have since spoken twice. Menocal has not changed Amaya's mind, but he said he understands her frustration: "She mistakenly thinks there is something of value that her son could be given but he can't get it."

The Larry P. case was filed in 1971 at the urging of black psychologists and others who discovered the state's special-education classes were 27 percent black -- three times their proportion in the normal school population.

In a six-month trial, several expert witnesses argued that many blacks had been placed mistakenly in the classes for slow or erratic learners. They said the IQ tests used in selecting them had mistaken their cultural disadvantages, such as ghetto slang and lack of books at home, for low intelligence. This doomed them to years of being branded "retarded" and kept out of classes that matched their real abilities, they said.

Although Peckham temporarily banned the tests even before his 1979 decision, few parents noticed it because, until the state gave up fighting the suit last year, many school districts whose administrators wanted to use the tests "were creating loopholes you could drive a truck through," Menocal said.

California school officials, as well as those around much of the country, still disagree over use of the tests. NAACP leader Clark, in his letter to Amaya, seemed to blame the Fontana Unified Schools for unnecessarily irritating her: "The school system should develop a more sensitive means of informing parents of the Larry P. decisions and its intentions rather than a form letter with special emphasis placed on a two-line postscript."

A spokeswoman for state school Superintendent Bill Honig said he considers the tests banned by Peckham to be among many "legitimate assessment tools" but will comply with the ruling. Honig's deputy superintendent for specialized programs, Shirley Thornton, said she thinks that Peckham "made a good decision from my perspective as a black educator." She is working with a state task force to determine what other methods, such as close classroom observation and tests based on actual school lessons, can be used to assign students to special-education programs.

Jerome M. Sattler, a psychology professor at San Diego State University who testified in favor of the tests at the Larry P. trial, considers that approach "absurd." He said he believes that no test or other selection process can find some core of intellectual ability free of environmental factors. Intelligence and all other parts of a child's personality come from "the interaction of both genetic and environmental factors," he said. "There is no way to isolate either of them, and it is stupid to try to do it."

The proportion of blacks in special-education classes should not be blamed on biased tests, he said, but on the fact that unusual numbers of black children grow up in poor families and, like poor white children, often have educational problems that require special help.

Sattler has offered to help Amaya, as has a local Republican Party group with a pledge of $200 for legal expenses she might incur. Her most potent new ally appears to be Harvey Mudd College government Prof. William B. Allen, appointed in April by President Reagan to a nearly six-year term on the U.S. Commission on Civil Rights.

Allen said Honig's office has promised him the results of an internal review of California's IQ-test policy within a week. Allen said he plans to discuss the Amaya case at the July 13 commission meeting in Washington. He said that Amaya's rights as a parent appear to have been violated and that the state should find a way to give those who want the test the option of taking it.

Test opponents said the ban is similar to the constitutional prohibition on teaching religion in public schools, even if parents request it. Allen called that "a false analogy."

"Religion is not used to determine a child's educational needs," he said.

To Amaya, the entire debate over race and its impact seems false and irrelevant, at least for most of California today. She pointed to her racially mixed family, and beyond. "You can't go into any neighborhood anymore and call it a white middle-class neighborhood. There is no such thing."