Efforts to raise the academic quality of American schools are unlikely to have a long-term effect unless the schools also take major steps to develop good character in their students, a group of scholars and educators warned yesterday.

As academic performance fell over the past two decades, signs of "serious forms of youth disorder" increased, the group said, citing soaring rates of teen-age suicides, homicides and out-of-wedlock births.

"Many schools . . . ignore character development and the formation of cooperative attitudes and skills," the group said in its 35-page statement, even though such traits as "persistence, cooperation, and moral integrity are central to individual . . . success" and community well-being.

The 27 signers of the "Thanksgiving Statement" include political conservatives and liberals, university professors and school administrators. Among them are Cornell University psychologist Urie Bronfenbrenner, a pioneer in the Head Start nursery program; Harvard University professors James Q. Wilson and Nathan Glazer; Paul De Hart Hurd of Stanford University; and school superintendents David A. Bennett of St. Paul, Minn., and Francisco D. Sanchez Jr. of Albuquerque, N.M.

Its editor is Edward A. Wynne, an education professor at the University of Illinois at Chicago who has done extensive research on character formation, including a sharply critical study of child-rearing and education in the suburbs.

Among the recommendations in the report are:

*More emphasis on group projects and academic team competitions

*More student responsibility for discipline and the upkeep of the school building

*Community service projects

*More "ceremonial activities," such as assemblies and opening exercises, that emphasize values that schools wish to promote, including cooperation, effort and patriotism.

The report suggests, as a way of encouraging students to develop stronger relationships with particular teachers and small groups of other students, reducing "departmentalization" in junior and senior high schools under which pupils are regrouped for different subjects.

"Good character is not generated solely by more homework, rigorous traditional grading and better pupil discipline," the statement said, adding that "continuity of relationships and knowledgeable adult direction" are essential.

According to federal statistics cited in the report, the rate of out-of-wedlock births to white females aged 15 to 19 has risen by 800 percent since 1940; the rate of death by homicide for white males, 15 to 24, climbed 315 percent from 1955 to 1981, while the suicide rate for the same group increased by 238 percent during that period. Wynne said the statistics for blacks showed somewhat different trends but also were "very distressing."

Wynne said the group discussed new curriculums that try to teach character and values, but decided not to recommend any. "In general, character is conduct," he said. "We believe the organization of the school is more important in developing character than the curriculum."

In an interview, he said the recent highly publicized rise in teen suicides probably stems not from increased pressures but from a "general weakening" of the insitutions that develop character and "rootedness" in young people: the family, neighborhoods, religion and the schools.

The report also calls for more demanding textbooks; higher academic standards, particularly for bright students; and "career ladders" for teachers -- steps that many states have adopted or are considering.

A majority of the signers endorsed tax credits or other private- school subsidies, but this concept drew strong dissents from four participants who said they favored giving students more choice among public schools but opposed public support for private education.

Wynne said the statement, entitled "Developing Character: Transmitting Knowledge," was financed by $10,000 in private grants, the largest from the J.M. Foundation of New York and the Johnson Foundation of Racine, Wis.

Other signers include psychoanalyst Ernest van den Haag; Stanley Elam, former editor of Phi Delta Kappan, an education magazine; and Michael Novak of the American Enterprise Institute.