Nearly 900 students might have been buried in the ruble in Sichuan province, Xinhua News Agency reported. It did not immediately give any other details or say if any of the students were thought to be alive.
Local media said a "row of houses" had collapsed in a city near the quake's epicentre.
The quake was powerful enough to sway buildings as far away as Beijing, Shanghai and the Thai capital, Bangkok.
Damage
A statement issued by the local government in Aba prefecture, where the epicentre was located, said buildings had cracked and collapsed, and mountain roads were damaged.
The area has a population of about 110,000 residents, the state-run Xinhua news agency reported.
Wen Jiaboa, China's prime minister, has announced that he will visit the affected areas. He has also ordered for troop deployment to carry out rescue and relief.
Al Jazeera's China correspondent Melissa Chan said the decision by the prime minister to rush to the area indicated the severity of the earthquake and the damage it may have caused.
A magnitude 7.5 earthquake is considered a major event, capable of causing widespread damage and injuries in populated areas.
A man who answered the phone at a branch of the Holiday Inn hotel in Chengdu said the quake lasted about five minutes and there were several aftershocks.
Aftershocks
"We all felt very scared," he said, but added that he had not heard of any major damage such as collapsed buildings or casualties.
Jessica Sigala of the US Geological Survey based in Colorado said the quake struck at a relatively shallow depth, so aftershocks could be expected.
She added that quakes of magnitude 7.5 do have the potential to cause significant structual damage.
China's deadliest earthquake in recent memory hit the eastern city of Tangshan in 1976.
The magnitude 7.8 earthquake followed by an equally powerful aftershock almost a day later killed around 240,000 people.