They danced, discoursed about defense and studied Spanish at Montgomery Mall last week to show off Montgomery County's public education system.

Hundreds of students took part Friday and Saturday in the fifth annual education fair, an event staged to demonstrate special programs and innovations of the school system to adults who might not be aware of what their tax money goes for.

"We do it as a way to show people who don't have children in the school system what is going on there," said Sally Keeler, fair coordinator. "Only 30 percent of the adults who live in Montgomery County have children in the schools, but they all pay the bills for what we have."

African folk dances, choruses and dramatic interpretations were performed in the middle of the mall; science and art projects filled the aisles in front of clothing and sporting goods stores. Classes in French and Spanish were conducted periodically through the day.

The mixture of sounds grew loud as the day grew long Friday. Shoppers stopped to watch the dancers. Preschool children, holding their mothers' hands, peeked at little tinfoil robots made by elementary school students.

Amid the hubbub, Brian Campbell, an eighth-grader at Parkland Junior High School, found a moment to discuss defense, destruction and the science project he produced for his honors class.

"Stars Wars, the system that President Reagan wants, would be 98.8 percent effective against missiles," he said, pointing to a light display he created -- in four days -- that he said explained the system. "But that's still not enough. That means that two out of a thousand warheads could get through. That's two cities that could be gone." He paused for a moment, and added,"That's far from perfect."

Campbell's display, one of about a dozen from Parkland, was clearly one of the more serious entries in this year's fair. Others, like the one entered by classmate Sam Rankin testing the effectiveness of detergents, appealed to consumers wandering the malls.

Rankin tested the veracity of the commercials he saw on television. He tried All, Wisk, White Magic, Bold and Cheer on a cotton rag stained by tea, jelly, mustard, lipstick, whisky and ink. "My mom helped me decide what did the best," he said. "All got it all."

Parkland teacher Clare Gaffigan, an 11-year veteran of the school system, said she asked her students to investigate subjects that interested them. Many wanted to test consumer claims, she said. "After 11 years, I'm not surprised by too much," she said, " . . . but I did have one girl who called Colgate to tell them that 'despite what the others claim, my survey showed you beat Crest.' "