I left my air-conditioned office yesterday and took an air-conditioned Metro train to check on my 9-year-old son at his un-air-conditioned D.C. public school, which was letting out early because of the heat. When I walked into his third-floor classroom, I saw a lot of faces glistening with sweat.

"It's not so bad," my son said, no doubt setting the stage for permission to play outside once he got home.

Still, it was odd that the only place I'd visit this day that did not have air conditioning was a school. Grocery stores, movie theaters and the building at 825 North Capitol St., which houses the D.C. school superintendent's office, were all nice and cool.

I went to the National Building Museum, which was featuring an exhibit, "Stay Cool!," on how mechanical climate control had transformed life in 20th-century America. Too bad so many of our public school classrooms have been left behind.

"The heat makes it hard for the children to concentrate, especially on writing assignments," my son's teacher said. "They become dehydrated from sweating and lethargic because it is so warm and humid."

Conventional wisdom holds that the reason only 30 D.C. public schools have air conditioning is that the school facilities are so old and need other repairs first. Just last year, however, a place as

old as Mount Vernon -- George and Martha Washington's 18th-century mansion -- was fixed up and air conditioned.

I say, if a dead president's house is worth it, surely the houses of learning for our children should be.

Of course, there will be some activist parents at my son's school who will read this as yet another media morale buster. Why don't I just chip in and help buy some air conditioners instead of carping all the time, is the way they see it.

Well, I do chip in. Plenty. District taxes are among the highest in the nation, and it seems to me that my child ought to be able to spend an entire school day in school.

We have only 71,000 students in D.C. public schools, far fewer than most big cities. And yet, the District's record of caring for them is among the worst in the nation. No air conditioning in public school classrooms is just another reminder of our institutional and systemic disregard for children.

After all the studies and task forces and commissions and reports, emergency school boards, control boards and superintendents over the years, the day still boiled down to children being sent home because of inadequate school facilities.

In Hawaii, where my son's teacher came from, unscheduled school closings are virtually unheard of.

"The only time I can recall public schools being closed in Hawaii was for a tsunami warning, which is a tidal wave," she said. Here, all weather -- sun or snow -- is tsunami.

I noticed that one classroom had an air-conditioning unit, apparently because it once housed the school's computer science program -- and those computers needed a controlled climate. Now the program is in another room, and the air-conditioning unit has gone on the blink.

"Are you all cool?" I asked the teacher.

"No," she said. "It doesn't work. It conks out after 10 minutes in this kind of weather."

Before air conditioning, life in America followed seasonal cycles, with indoor productivity declining in proportion to the heat and humidity. As the "Stay Cool!" exhibit noted, when workplaces and classrooms got too hot, people were sent home.

But that was supposed to have been in the old days.

In my son's school lobby yesterday, I met other disgruntled parents arriving to pick up their children. Some had to take off from work and now would have to scurry to find babysitters.

"They [school administrators] knew it was going to be too hot for school," a parent fumed. "Why did they wait until we got here this morning to tell us that there would only be a half day?"

The exhibit at the Building Museum was supposed to show how far we had come in our quest to control weather. There was a photograph of an ultramodern 1,100-ton "chiller" that the J.C. Penney Co. uses to keep its home office employees comfortable. The Solar Energy Research Facility at the U.S. Department of Energy has developed all manner of solar-powered cooling devices.

The technology exists to take Washington's record heat and make ice cubes out of it.

Instead, rivulets of sweat poured down the steamy little faces of hundreds of D.C. public school students as they were forced to leave school early.

In 1893, R. Ogden Doremus, noting the use of iceboxes at slaughterhouses, made a case for air-conditioning Wall Street.

"If they can cool dead hogs in Chicago," he wrote in the North American Review, "why not live bulls and bears on the New York Stock Exchange?"

More than a century later, I'd like to know: If they can cool school administration offices, why not the classrooms, too?