Acting D.C. Schools Superintendent James T. Guines said yesterday the city's swiftly implemented midyear promotion plan is "stupid" and unnecessarily punitive toward students, and should be revamped if the trend toward massive failures is not reversed by the end of the year.

"Frankly, it scares me what we've done to our first-, second- and third-graders," Guines said of the students in the grades where the new promotion standards, called the "pupil progress plan," are currently in effect. Last month, about half of the city's 21,600 first-, second- and third-graders were not promoted for failing to achieve required math and reading skills.

In statements obliquely critical of former Superintendent Vincent E. Reed's Administration, Guines also said he believes the progress plan was poorly implemented and works "totally contrary" to the way in which youngsters learn.

Despite Guines' critism of the plan, he was chiefly responsible for its development and implementation under Reed. Guines and Reed never were close professionally or personally.

The new promotion standards are part of an overall school system effort to raise the achievement of city students who consistently have scored far below national norms in reading and math.The plan was at first popular among parents and school board members, but with so many students failing many parents have complained about it.

The progress plan calls for students to be promoted on a semester basis and only if they have mastered certain reading and math skills.

Although Guines made public presentations in support of the Reed administration's program, he said yesterday that he never personally supported the Reed plan. "Now that I am acting superintendent I have no problem addressing how the implementation was wrong."

Guines said that when he was associate superintendent for instruction under Reed, he privately supported a plan that would have basically eliminated the traditional elementary grades and placed youngsters in two "instructional blocks," a primary block corresponding to the first four years a child is in shcool and an intermediary block, corresponding to the following three years.

Under that system, youngsters still would have to master certain specified skills in reading and math, but promotion would not be an issue until the end of the child's fourth year, when students would be tested to determine if they were ready to enter the intermediary block.

This way, Guines, said teachers would have three years to bring students up to grade level. With semester-by-semester promotions, Guines said, both teachers and students are under too much pressure to master the required skills within too short a time.

This is the plan Guines says he would propose to the board to replace the Reed plan, if so many students continue to fail.

Neither Guines nor other school officials could name another school system in the country that has the kind of system he says he proposed.

Guines took the job of acting superintendent after agreeing with the school board that he would step down July 1, but he since has expressed a strong interest in staying on. He stopped short of saying Reed was wrong in the way he allowed the pupil progress plan to be implemented.

Guines would say only that the "former leadership" of the school system had a mentality geared more toward secondary education, where many courses are tought on a semester basis. This obviously was a reference to Reed, who had been superintendent of secondary schools prior to assuming the top past in 1975. He added that Reed had the final say on the form the pupil progress plan did take.

Reed declined to comment on Guines' statements but said, "I support the student progress plan and the board of education accepted it."

Joan Brown, director of special projects and programs who was on Guines' staff at the time these decisions were being made, said Guines' proposal to create instructional blocks was "philosphically the best idea for making this plan work . . . it would have removed the stigma associated with repeating [a grade]." She said, however, that many school officials were reluctant to do away with traditional grades. No large city school systems in the country have completely ungraded elementary schools, according to one national study.

"If [the current] plan works, I'm committed to leaving it as is," Guines said.

Brown said the committee overseeing the pupil progress plan will consider next month whether the system should extend the promotion standards to all elementary grades. There is concern, she said, that even larger numbers of students would not be able to master the required skills and the system would not be able to give them the remedial help they would need to catch up. Right now, the school system is trying to recruit 1,000 volunteer tutors to help about 6,000 youngster who failed both reading and math at their grade level.