The explosion yesterday damaged an outdoor-reading area of the San Kamphaeng district school. No one was injured.

Police and military officers questioned two Grade 8 schoolboys for more than an hour.

Investigators recovered bomb debris including electronic circuits, a clock face and a coffee tin.

Security officials cordoned off the school immediately after the explosion fearing subsequent blasts. Afternoon classes were cancelled although no more bombs were found. Military vehicles guarded the school.

Lt-Colonel Noppakhun Kiratikarnkul of the Chiang Mai police said evidence suggested the bomb was home-made and designed to be detonated by remote control.

The two students told officers they received the device from a Chiang Mai vocational-school student, planted it and set the timer.

Police searched the students' homes. They remained concerned at the complexity of the bomb.

Headmaster Seri Suwanpetch said it was fortunate the explosion happened when students were in class.

He was convinced it was a prank and not malicious.