CHANGSHA - A bus loaded with a 48 school?students on its way to sit an exam fell off a 30-meter-high slope in Yongzhou City in central China's Hunan province Thursday afternoon, leaving at least four people dead, local authorities said.

The accident happened at 5:50 pm, when the bus -- which has a capacity of 49 and was loaded with 46 students and one teacher from Hunan Daoxian County Junior Normal College, along with the driver -- was on its way from Daoxian to Yongzhou, sources told Xinhua.

Two students died at the scene and another two died after being sent to hospital, said an official with the Yongzhou municipal government in a deep-night press release.

Three people remain in the intensive care unit (ICU) and five others including the driver are also badly injured, the official said, adding that the rest of the passengers all suffer slight injures.

All the injured are receiving treatment at the affiliated hospital of Yongzhou Vocational Technical College.

Ouyang Ping, a 21-year-old survivor, recalled the "horrible" moment of crash. "I suddenly saw branches and leaves out of the bus window, and I realized something was wrong. The bus crashed to the ground before I had time to protect my head. We were all upside down."

"People were screaming. It was horrible." Ouyang sat at the back of the bus and managed to climb out through the window. He then helped two other students out.

"I didn't realize I lost one of my teeth until I climbed up the slope," he said, with his mouth swollen.

The students, aged between 17 and 21, from the junior college with 3,000 students, were heading to Yongzhou as scheduled to sit the higher education self-study exam, said Mo Junxue, an official of the college office.

A total of 171 students were headed to Yongzhou for the exam, in four rented buses accompanied by four teachers, and three of the buses arrived safely in Yongzhou, Mo told Xinhua via phone in an exclusive interview.

The exam was scheduled for Friday and the college planned to let the students stay in Yongzhou on Thursday night, Mo said.

A faculty team led by the college president has arrived at the scene, according to Mo.

"No collision happened. It might be a brake malfunction, we guess," Mo said.

Rather than rolling off the slope, the bus slid off it when turning a corner, said a restaurant boss who witnessed the accident.

The bus was then visible upside down at the bottom of the slope, witnesses said.

Yongzhou government officials have also arrived for rescue and investigation. The press release from the municipal government blamed poor driving for the accident, saying further investigation is still underway.

The teacher on the bus, named Deng Guolin, 31, was hurt on the head and has been in a coma, said a family member of Deng who answered his phone.

"Doctors said he needs rest now," said the man.

Local police soon arrived at the site and local residents joined search and rescue work right after the accident.

"The college official said there was one student missing. All the people here joined the searching but later the official said they got the wrong number of students," a traffic policeman at the scene told Xinhua via phone.

The rescue work had ceased as of 10:30 pm and a crane was sent to drive away the bus, witnesses told Xinhua.