The months of March, April and May are typically the hottest and driest in the archipelago nation, but conditions this year have been exacerbated by the El Nino weather phenomenon.
Many schools have no air-conditioning, leaving students to swelter in crowded, poorly ventilated classrooms.
"Even my smartest student is not in the mood to answer qunestions because it's very hot," said Mayette Paulino, who teaches a grade two class of around 27 children near Manila.
She said "students feel tired and seem sleepy" as the heat intensified in the afternoon.
The Department of Education, which oversees more than 47,000 schools nationwide, has issued an advisory giving school heads the power to decide when to switch to remote learning "in cases of extreme heat and other calamities".
Official figures for Friday showed 5,288 schools had suspended in-person classes, affecting more than 3.6 million students, after 4,769 schools on Thursday.
Nearly 300 schools in Manila alone had remote learning on Friday, while the central regions of the main island of Luzon were the hardest hit, with more than 1,600 schools suspending in-person classes.
Some schools have reduced class hours to avoid teaching during the hottest times of the day.
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