Indonesian rescue workers were racing against the clock on Wednesday in the search for survivors from a school collapse in the province of East Java, with at least 91 students still unaccounted for, along with three confirmed dead and about 100 injured.

The Islamic boarding school, which authorities said was undergoing an unauthorized expansion to add two new stories, collapsed during afternoon prayers on Monday, sending slabs of concrete and other heavy debris crashing onto the students below.

Most rescues typically happen within 24 hours after such a disaster, with chances of survival decreasing each day after that, and more than 300 workers continued to work desperately at the scene to try and reach those who have been detected to be still alive and trapped below.

Of the approximately 100 injured, 26 are still hospitalized and many are said to have suffered head injuries and broken bones, authorities said.

Rescuers have been running oxygen, water and food from narrow gaps to those still trapped under the debris to keep them alive.

Search teams have also used detectors and thermal drones to detect potential survivors who could be rescued.

The structure fell on top of hundreds of people at about 2:30 p.m. on Monday in a prayer hall at the century-old al Khoziny Islamic boarding school in Sidoarjo, on the eastern side of Indonesia's Java island.

The students were mostly boys in grades seven to 12, between ages 12 and 18. Female students were praying in another part of the building and managed to escape, survivors said.