Claiming himself to be a leader of an underground outfit, a man phoned Abu Sayeed, headmaster of the school, and said they would launch grenade or bomb attack on the reputed educational institution by July 31.

The unknown caller, however, did not name his political organisation, the headmaster said.

On June 17, he received similar threat in a letter sent to him, which he ignored, he added.

English teacher Mohammad Mosharaf Hossain recently left the school after unknown criminals threatened to kill him for failing to pay them extortion money, Sayeed said adding that two other teachers are also fearing attempts on their lives as they also refused to pay tolls.

No case was filed till 6:00 yesterday evening.