The blaze destroyed a dormitory at the school in the mining town of Mahdia on Monday.
Investigators said initial findings suggest that it was due to a malicious act.
President Mohamed Irfaan Ali has decreed three days of national mourning.
Police acknowledged they do not understand the motives of this possible intentional act.
The doors were locked and the girls could not escape because the windows were barred.
Firefighters had to open holes in the walls to rescue the survivors. Seventeen people are hospitalised.
