The state-owned Santa Teresa de Valencia opened its doors on Monday to towel-toting pupils dressed for a day at the beach.
Similar scenes took place at Almoines and Carcaixent last week, while other schools moved lessons outdoors and closed early.
Jordi Boluda, a board member of Santa Teresa's parents' association, described the unusual uniform as "a fun protest" to draw attention to the 30 to 31ºC temperatures in the school's classrooms.
The school is new but, according to Spanish daily El Mundo, has design problems including large windows that turn the classes into "ovens".
"We can't be like this, with 30º in the classrooms like it's been all week," said Boluda.
The issue of overheating classrooms led to disagreement between different departments of the regional government.
Education councillor, Maria José Catalá, expressed his concern that some schools might not reach the minimum of 180 annual teaching days if forced to close regularly due to high temperatures.
But his department insisted that air-conditioning systems need not be fitted in public schools and that "natural" cooling methods, such as opening the windows, should be used.
Air conditioning is fitted to some prefabricated classrooms where temperatures can be even higher, but the regional health department said that there was "no current health regulation preventing the installation of air-conditioning in schools."
Unions pointed out that the maximum legally permitted temperature that teachers, as employees, could be expected to tolerate was 27ºC.