Smellier pupils are among the consequences of global warming, the NASUWT conference heard.
"When classroom temperatures soar up to the 40s, you begin to hum a little," said Mick Mumford, from Hampshire. "And you've got 30 teenagers in there who've hummed since they woke up."
Delegates voted to campaign for a legal maximum temperature for classrooms.
The NUT said teachers should walk out if the 26C barrier was broken for more than "very short periods".