According to police, the boys pulled a branch of a mango tree when the bus, carrying 62 students on a tour of some tourist spots, stopped near a tea shop.
However, the branch hit a dry frond of a nearby coconut tree, and in the impact, the frond fell on the high tension wire snapping it.
While two boys Mukesh and Karthick Selvam, aged 15 and 16, died on the spot, 27 boys and girls suffered burns.
Thirty-three others were not injured as they managed to alight immediately, police said, adding, locals put out the fire. The bus was partially damaged.
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State Finance Minister O Panneerselvam rushed to the spot and made arrangements to send 20 boys and girls to the government hospital at Periyakulam while seven others, who had serious burn injuries, were sent to the government medical college hospital at Theni, officials said.
This year they had taken the students from Achampathu for a tour of the Sothuparai dam and other tourist spots.