The girl, who has not been named, received the burns on the thigh and back when her iPhone started smouldering in her pocket during the first period of her school day, which happened to be French class.
The teenage student of Middle School of the Kennebunks in Maine was treated at a Southern Maine Health Care hospital in Biddeford on Friday, and reportedly released after about 45 minutes.
The girl said her pants started burning after she sat down with the phone in her back pocket and heard a pop, principal Jeffrey Rodman was quoted by the New York Daily News as saying.
Rodman added: "Immediately, smoke starts billowing from around the student. She knew right away something was wrong and, in a panic, knew her pants were on fire caused by the cellphone. It was sensitive. She knew she was kind of in a tough situation."
Rodman said the student quickly raced to the corner and took off her pants. A teacher came to her rescue, wrapping her in a blanket before rescue workers arrived on the scene. The classroom had to be vacated, and the student was treated for minor burns.
Rodman said the girl's pants had multiple burn holes and there was so much smoke in the room that a window had to be opened.
He said the cause of the phone's explosion is under investigation by the state Fire Marshall.
Written with inputs from PTI