The National Education Association warned yesterday that a shortage of qualified teachers is prompting school districts to increase class sizes, reduce course selection and rely on teachers' aides and other uncertified employes to teach.

The warning was based on a survey of 128 of the nation's 190 largest school districts and released by the teachers' organization at its annual meeting of 9,000 delegates in Los Angeles.

According to the survey -- commissioned by the NEA and conducted by Commercial Analysts of New York, an independent survey firm -- nearly 35,000 teaching vacancies remain for the coming school year, an 8.2 percent increase over the number reported at this time last year. More than half of the districts said they expect enrollment increases.

To fill those teaching vacancies, 48 percent of the districts said they will use temporary or substitute teachers, 41 percent said they will issue emergency certificates to people not trained as teachers and 40 percent said they will assign classroom duties to teachers' aides and interns, according to the survey.

"I think we're looking at a tremendous problem," said Keith Geiger, NEA vice president. "In five years, teachers will again be blamed for the quality of education, which will be lower."

According to the survey, nearly 40 percent of the districts said they will assign teachers to teach outside their fields of study, 35 percent said they will reduce course offerings and 19 percent said they will increase class size.

The NEA, which has more than 1.8 million members, has for several years predicted a critical shortage of teachers, citing the need for improvements in salary and status to draw more young people into the profession.

There is conflicting evidence, however, on the extent of the teacher shortage.

Last week, a Washington-based education group released a study showing that public school districts have enough applicants to fill their teaching vacancies this fall. Based on a survey of 93 school districts, including the largest 16 systems in the country, the National Center for Education Information reported that many districts are receiving an unexpected surplus of applicants.

Also, the U.S. Bureau of Labor Statistics reported last year that no large-scale teacher shortages are likely in the next decade.

NEA executive director Don Cameron argued that, despite what may be sufficient numbers of applicants, the supply of qualified teachers is inadequate.

"If the public allows school districts to hire anybody off the street, of course we won't have a teacher shortage," Cameron said.

The NEA said the most acute shortage is in bilingual education, where 80 percent of the positions are unfilled. Nearly half of the positions in special education and a third in science and mathematics are open, the survey said.

The organization has also warned of a shortage of minority teachers. While the percentage of white teachers climbed, the percentage of black teachers declined from 7.8 percent in 1981 to 6.9 percent in 1986, according to the NEA.

If present trends continue, minorities will make up a third of public school enrollments but only 5 percent of teachers.

"That sends a terrible signal to all students -- the signal being we are not attracting minority students into the most nobel profession," Geiger said.