According to the passage, if all students attend year-round schools in U.S. they

Most children in the United States have a summer holiday of almost three months mostly because it was difficult to hold classes in the hot summer weather.
    American schools began to experiment with changing this system. One idea was for children to attend school all year round. (11) Students who attend year-round schools in the United States do not go to class every day of the year. But the time is organized differently. For example, some students attend school five days a week for nine weeks. Then they have three weeks off before going back to school for another nine weeks. In the summer they have about five weeks of holiday.
    Supporters of year-round schooling say it improves student learning. They say students in the traditional system often forget what they have learned during a long summer holiday. They say schools can offer special classes during the short holidays for students who need extra help.
    (12) People opposed to year-round schooling say the research has not proved that it improves learning. Families report problems organizing activities and travel when children in different schools have different holidays. Some parents want their children to have an extended summer away from the pressures of school. People who operate summer camps for children oppose the idea. So do businesses that employ teenagers in the summer.