If all goes according to schedule, more than 900 children will begin classes early next month in the nation's first 100 per cent solar heated and cooled school, located inside a hill in the new town to Reston.

"This technology really started with the cavemen. We got away from it when energy was so cheap. Now we're going back," said Fairfax County's school construction director Alton C. Havin, standing at the crest of the hill, directly above a classroom buried in the earth beneath him.

In its most fundamental terms, Hlavin explained, the project simply represents a return to using the natural heat-insulating properties of the soil and the warming power of the sun to conserve energy.

To do that, school builders essentially bulldozed off the top of the hill, poured a concrete shell for the shell for the school, then put the hill back again, as if it were a hat on the school's roof.

Then, with a $665,000 grant from a university in Saudi Arabia, they designed an elaborate system for heating and cooling the school with solar energy using a computer an 4,860 glass solar collectors to capture the sun's heat.

The project had its origins during the winter of 1973-74, with the energy crisis and the Arab oil embargo. Hlavin and his staff were seeking ways to save energy and eventually they hit upon the idea of putting a school underground and using the sun to heat and cool it.

"At first we named it the 'underground school,'" Hlavin said, "but that sounds kind of gloomy. Over drinks at lunch one day, we came up with the name Terraset,' meaning set in the earth."

Intially, Hlavin applied to the National Science Foundation for a grant to heat the school with solar energy, but he was turned down. A few weeks later an agent who turned out to be a Saudi representative approached him and asked if he'd be interested in money from a private source.

Hlavin said yes, and the deal was later sealed between the Fairfax County schools and Saudi Arabia's University of petroleum and Minerals.

Under the agreement, the Saudis are entitled to the benefit of any technological discoveries and have the right to visit the school. Their interest in the project, Hlavin explained, stems from the fact that their oil resources, vast as they are, are limited and will eventually run out. Sunlight, which they also have in abundance, will not. Hence the impetus to develop uses of solar energy.

So, while the pace of business elsewhere in the area slowed during the holiday season, a corps of workmen labored feverishly to complete the finishing touches in Terraset and meet the Feb. 1 opening deadline.

Over a sunken courtyard at the front entrance to the building stood a 13,000-square-foot iron rack where the solar collectors will be installed.

The solar system will operate via 4,822 feet of tubing through which thousands of gallons of water being warmed directly by the sun's rays and indirectly by reflectors will be pumped. Computers will regulate the temperature between 180 and 240 degrees - the system is pressurized, enabling the water to reach temperatures hotter than the 212 degree Fahrenheit boilding point.

Passing from the solar collectors, the water flows into two 2,500-gallon storage tanks and from here - if it is being used for heating purposes - to heat the water in heat the water in the school's hotwater heating system.

If it is being used to cool, the building, the hot water - at 240 degrees - is cooled through a process of compression, condensation and absorption that brings it down to a temperature of 42 degrees. It is then circulated through coils and fans blow the air past the coils, thereby cooling the building.

As a safeguard against sunless days, there are three 10,000-gallon insulated storage tanks that can hold water without the temperature changing more than one degree a day plus a backup electric-boiler system that Hlavin estimates will be used no more than two days a year.

In a normal Fairfax County school the size of Terraset, heating costs run to about $40,000 a year, Hlavin said. At Terraset, the costs will be about $10,000, and most of that will be to run pumps and fans, he said.

The building itself is completely earth covered on top and on one side, leaving three sides with windows from which children can look out on the hillside. Inside there are offices, a library, gymnasium, cafeteria and four circular "pods," each of which can be divided up into eight pie-shaped classrooms. The earth covering on top ranges from 2 to 8 feet thick and eventually it will be landscaped and used as a play area.

Since Fairfax County first announced plans to build Terraset - at a cost of $2.7 million - more than two years ago, officials have had about 400 requests for information about the school, ranging from the Israeli Department of Agriculture to education officials in Sweden and Denmark.

Closer to home, there are some skeptics.

Around the state of Virginia, when Hlavin attends meetings of fellow school construction officials, there is a stock question.

"Have you gone crazy up there in Fairfax? Are you going to bury all your school?"

No, Hlavin usually answers. "But I do hope we're going to lean something from Terraset."