This month millions of American kids can forget about classroom bells and set off for grandparents ’ homes, sleep-away camps and lifeguard stands . But summer vacation hasn ’ t always been a birthright of the U.S. schoolchildren. Before the Civi l War, schools operated on one of two calendars ( 日历 ) , neither of which included a summer vacation. Rural( 农村的 ) schooling was divided into summer and winter terms, leaving kids free to help with the farm work in the spring planting and fall harvest seasons. students, meanwhile, regularly had as many as 48 weeks of study a year, with one breaker quarter. In the 1840s, however, educational reformers like H o race , Mann moved to combine the two calendars out of concern that rural schooling was not enough and that overusing of young minds could lead to nervous disorders. Summer appeared as the obvious time for a break: it offered a rest for teachers, fit in the farming calendar and reduced doctors' concern that packing students into hot classrooms would promote the spread of disease. But peoples opinion about the modern U.S. school year, which averages 180 days, is still divided. Some experts say its pleasant but lazy summer break, which took hold in the early 20th century, is one of the reasons math skills and graduation rates of U.S. high schoolers ranked well below average in two international education reports published in 2007. Others insist that with children under increasing pressure to devote their downtime to internships ( 实习 ) or study, there's still room for an institution that protects the lazy days of childhood.