The US government has decided to block a private shipment of school supplies and educational kits to Cambodia as part of its effort to deny ''development aid'' to that Southeast Asian country.
The relief agency of the Mennonite Church, which has proposed sending the educational materials, is appealing the decision. The Mennonites had managed to reverse an earlier government decision to block a shipment of wheat flour destined for malnourished and handicapped Vietnamese children.
This time, State Department officials indicate that a reversal is unlikely. An official said that US policy is to allow humanitarian, or emergency, aid to go to Cambodia but not to allow development aid, or aid which would help the Vietnamese-supported Cambodian government to ''institutionalize'' itself. Following Vietnam's 1979 invasion of Cambodia, the US pressed international agencies to reduce their aid to Vietnam and tightened licensing procedures for private aid to that country and to Cambodia.
That Vietnamese invasion drove from power the Pol Pot regime, which has earned a reputation as one of the most brutal tyrannies in recent history. Under Pol Pot, hundreds of thousands of Cambodian civilians were killed, including many school teachers. The new regime in Phnom Penh has had to start virtually from scratch in the educational field.
The Mennonite aid consists of Christmas kits being put together by Mennonite families throughout the US and Canada. Each kit includes a homemade denim bag, two pencils, two notebooks, an eraser, a ruler, and a ballpoint pen. The aim is to gather about 90,000 such kits.
''It's a people-to-people program,'' said Bert Lobe, secretary for Asia of the Mennonite Central Committee. ''It's an attempt to involve our people . . . and to help remind our nation that we have some responsibility for the situation in Cambodia.
''We are not the government,'' said Mr. Lobe. ''We're the people, and the Reagan administration is supposed to be an administration that is staying off the people's backs.''