A Bladensburg Elementary School class is suffering the sharp pangs of ill fortune. A slide show the students had hoped to show at a film festival in Dallas next spring has been disqualified because it is too sophisticated. Another contest they wanted to enter has been canceled.

The slide show lasts just three minutes, but has gone far. Last spring, the fifth graders' production came in first in the Prince George's County schools' film contest. It was one of three winners in statewide competition, where judges called the technique "dazzling." And it earned, everybody thought, the right to represent Maryland schools in the national competition in Dallas next spring.

But two weeks ago, fifth-grade teacher Pat Green learned the film cannot be shown at the National Student Media Film Festival. It is too sophisticated, and unless the children cut back from three projectors to two, and from three screens to one, they will be disqualified. The students decided their production would be downgraded too much if they changed it, and looked for another competition to enter. Harry Bock, audiovisual supervisor for county schools, suggested the children try a "multi-image" film contest being held in Dallas at the same time. But that competition was canceled for lack of entries.

The children based their film on the song "Sing a Song" from the television show "Sesame Street." They began the project by writing what they wanted to do and wrote away for permission to use the song.

"They studied it as a poem, and broke it down line by line, and illustrated it with three dimensional objects made of things like construction paper and wallpaper," the teacher explained.

"Thirty children were involved in it and they're disappointed," Green said. "Competing at the national level would give them a sense of ac complishment. We were really upset at the whole situation. Here we did something unusual, we feel, and have no outlet."

Last week several of the students involved in the project, now sixth graders, who had gathered to rehearse a showing to the PTA, spoke of their disappointment. "I was mad because we worked so hard and stuff," said 11-year-old Lisa King, one of the projectionists. "The whole class worked hard."

"The children didn't think they could do something like this," Green said. "They found out there's a lot of things maybe we could do if we just tried. They were so elated by the response. And then to have the film disqualified -- it was like having the carpet pulled out from under them. They found out that if you do your best, it's not always appreciated."

Disappointment is a jarring element in the otherwise outrageous cheerfulness of the 32-year-old Green's classroom. Last week the doors were guarded by two life-sized pumpkin-headed monsters and signs and pictures signaling the joys of life. A notice inside reads: "Welcome to Room 214, grade 5: The Green Scene. Mrs. Patricia Green -- The Boss. Coming attractions: . . . " Said King: "I've never seen her in a bad mood, but sometimes an allergy will get to her."

The prize-winning film makers are taking the good with the bad. "We're not going to let it discourage us," Green said. The lessons learned at each stage of the project are not forgotten.

Most of the pictures in the film are of cartoon characters dancing and singing and holding hands -- "showing how happy they are to be themselves," Green said. "It was a type of project that built so much self-confidence."

The students made the characters and then the pictures were photographed and the slides put in three projectors, to be flashed on the screen in time with the music. When the music begins, the three projectionists, using hand-held controls, bend their heads to look at the script, and tap their feet and swing their bodies in time with the music, quickly changing slides to match the lines of the song.

"Our music teacher, Mrs. Wilma Vazquez, she worked with the children and helped them learn the words for the song," Green said. "They practiced it over and over again." Three children played flutes at the beginning and end of the song.

"The singing is not, like, finished," pointed out Bladensburg Principal Frank Minni. But he was delighted by the show, he said, and proud of Pat Green and the children. "Super teacher -- super job," he said.

"It was hard, but it was fun," said projectionist Nicky Allen. Another projectionist, Postal Bumbray, agreed. "It taught us that we could do just about anything we want," she said.

Well, almost anything. When the children asked Green if they could show the slides at the school's talent show, she told them admission was being charged. "I said to the kids, 'You know we have a copyright law.' "

King remembered the class. "Some people said we should show it. They said 'Why don't we go on and do it; they would never find out.' " But after a long discussion, the fifth-graders agreed they shouldn't. "Again, that was a lesson," Green said. "There was some ethics in it. So after what happened on the national level, they felt whammied."

This year Green has a new class of fifth-graders and a new project. Each day a camera sits on a tripod in the classroom. "Any activity, they can take pictures," Green said. She wants the children to compile a history of their year in fifth grade.

"It's there and we can record the important things and the not-so-important things," she said. She doesn't know whether the class will enter any film-making competitions, but said it doesn't really matter. "They're learning a different way of looking at things. They notice so much more."