Public schools in the city are scheduled to open for another sweat-drenched day today, despite a call by the teachers union to consider closures or early dismissal.

This week’s unexpected heat is particularly unwelcome at public schools, as many lack air conditioning or have it only in some classrooms. Other schools have air conditioning in classrooms but do not have it in rooms where large numbers of students congregate in close quarters, such as the cafeteria and the gym.

The upper floors of school buildings — often accessible only by uncooled stairways — could bear the worst of the weather; the teachers union yesterday estimated that temperatures on those floors could rise above 100 degrees today.

At M.S. 131 in Chinatown, a janitor cleaning the school cafeteria, who had sweat through his undershirt, said he was expecting more air conditioners to be sent soon. “Can’t come fast enough,” he said.

“It’s awful,” a seventh-grader at M.S. 126 down the street, Ryan Roman, said. “We open up the windows, but that just makes it worse.”

Students said that the heat affected them and their teachers, too.

“When you’re hot, you get really cranky. When you’re cranky, you don’t want to do any work,” another M.S. 126 seventh-grader, Aaliyah Bodden, said. “We almost got in trouble today in this one class. The teacher gave out two detentions.”

Lack of air conditioning can also create unexpected side effects, such as excessive noise and walkouts.

The director of the Web site InsideSchools.org, Pamela Wheaton, said her staffers have watched streams of high school students simply walk out of their buildings on hot days.

The fans that many schools use to cool their rooms in place of air conditioning can be so loud they drown out teachers’ voices, she said.

All New York City public schools built in the last 20 years have central air conditioning. One hundred of the city’s 1,100 school buildings were built in the last 20 years.

The other 1,000 schools rely on individual window units, which, depending on the school, can be speckled across some classrooms, installed in almost all of them, or nonfunctioning.

A survey completed during the last school year found that 40% of schools had more than 26 rooms air-conditioned, while 93% had at least one room air-conditioned, school officials said.

A major problem is that many older schools do not have the kind of electrical wiring necessary to power full air conditioning.

School officials yesterday said the cost of upgrading city facilities to that level would be “extremely high.”

A Department of Education spokeswoman, Debra Wexler, said the city is asking its service and facilities employees to “stand by” to support principals for whom the heat is a challenge.

She said it has traditionally not been the department’s practice to close schools for heat or to dismiss students early.

Saying it fielded complaints from at least 13 schools yesterday, the teachers union chided the Education Department for not sending schools a strong enough “course of action” for dealing with the heat.

The city’s public advocate, Betsy Gotbaum, said: “It’s too bad the DOE is being less than proactive about soaring temperatures that are affecting city schools.”

Other area schools yesterday dismissed students early, including several in New Jersey, while districts across Connecticut shut school down altogether.

A longtime city educator who is the president of the Center for Educational Innovation, Seymour Fliegel, said closures and early dismissals do not make sense in New York City, where many children have no place to go but school.

Mr. Fliegel suggested other remedies, such as taking a class trip to the water fountain or letting every student rest his or her head on his or her desk for a few minutes.

He dismissed complaints from the teachers union.

“Maybe the union, in its next contract, since they always run out of things to put in their contract, will say, ‘No teacher will have to work on any day that is 98 degrees or higher,'” Mr. Fliegel said.