Fires at school dormitories are common in parts of Africa where there is little access to electricity and students study by candlelight at night.
Iringa's Regional Police Commander, Evarist Mangalla, said preliminary investigations indicate the fire was caused by a candle after a student fell asleep studying.
"We switch off the power generator after 10 p.m., when students wishing to continue their studies use candles or kerosene lamps," said the school's headmaster, Raymond Mlasu.
Tanzania's worst school fire in recent years was in 1994, when 43 girls died at a school near Mount Kilimanjaro, Africa's highest mountain.
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