Almost 70 percent of respondents also complained they are often banned from leaving the house after school to play or exercise with friends.
"I have to do my homework after school and have no spare time for exercise," said Shi Yang, a second-grade student at a middle school in Beijing's Chaoyang district.
Shi has attended extra classes every Saturday since she started as a senior, although she added that some elementary students get only half a day of rest on weekends.
One of those youngsters is Zhang Min's daughter. "I've signed her up for two weekend classes, dancing and English," said Zhang, who like many parents believe children involved in extra-curricular activities stand a better chance of getting into good schools and colleges.
More than two-thirds of the students interviewed by the CNIER last year said they spend more than two hours doing homework every day, with the same proportion also enrolled in after-school classes during weekends and holidays.
Zhang said she is fully aware of the pressure her daughter is under but "as there's such tough competition for good middle schools, she has to cope with it".
The entrance examination is too difficult for elementary school students because the content is beyond what the pupils are taught in class, she added.
Pastimes and diets
Health and education experts also criticize the increasing reliance on television, the Internet and games consoles, which many describe as "easy alternatives to exercise".
Before the widespread introduction of television 20 years ago, students used to indulge in outside activities and sport, but all that has changed.
Today, the three favorite things for young people to do is chat or play games online, listen to music and watch television, said Wu Jian, director of the CNIER's physical, health and arts education center, who is also a member of the panel responsible for organizing and analyzing the 2010 national physical condition survey.
"Their chief pastimes are now ones of inactivity," he said. And with living standards improving across China thanks to rapid economic growth, youngsters are now better fed than previous generations. However, nutritional experts say their diets leave a lot to be desired.
In the past decade, the average weight of Beijing students aged 7 to 18 has increased faster than the average height, reported the 2009 White Paper on Sanitation and Health.
In 2007, the Communist Party of China Central Committee and the State Council jointly issued a notice on strengthening physical education in schools and improving the condition of children. The regulations require primary and middle schools to organize one-hour group exercise sessions on school days when there is no PE class.
"The central government has put great importance on the physical condition of students but the key is implementation," said Huang Jin, who works in the youth department at the State General Administration of Sport.
Beijing's municipal authorities in 2009 announced that its standard of sanhao xuesheng, the Chinese term for honor students, will require excellent results in physical condition and health tests.
This year, for the first time, the PE section of the capital's senior high school entrance exam also featured a progress-based assessment that accounted for 25 percent of the final score.
However, Sun Yunxiao at the China Youth and Children Research Center said he feels the measures are "too utilitarian" and the risk is that, once students pass the exam, they will simply give up exercising.
"We should help students develop a good habit of exercise and a love of sport," he said.
Regarding diets, Sun suggested China learns from Japan, where primary and middle school students are provided with nutritious, balanced lunches of meat and vegetables that are specially designed by professional dietitians.
Meanwhile, the State General Administration of Sport is attempting to get more children involved in exercise nationwide by supporting 3,500 clubs to guide and organize students safely and scientifically.
"We hope these clubs will help students form one or two lifelong hobbies," said Huang.
However, schools should also be required to provide a good exercise environment to every student, said Mao Zhenming, dean of Beijing Normal University's school of physical education and sports.
"A criterion of sports facilities needs to be established for schools," he said. "It is still possible to reach a win-win situation of quality study and sufficient time for exercise."