Figure 1.--Prep schoolboys put on their best uniforms for church on snday. Notice the prefects wearvlong trousers. Also notice that caps were still worn at this traditional school.

English schools have had a variety of policies and regulations about the school uniform and wearing shorts. Beginning in the 1960s older English boys stopped wearing short pants as commonly as they once did, even for casual dress. Slowly British schools began to lower the age at which boys had to wear shorts and gradually most schools dropped the requirement all together.

Schools regulations on shorts varied over time. Shorts were widespread in many most primary schools and private preparory schools. Many grammar schools (academically selective secondary schools) and secondary modern schools required them in the first year or two. Even in the secondary schools that did not requite the junior boys to wear them, many boys did not, but this generally declined in the 1970s. Many public schools (private secondary schools) requird junior boys to wear shorts. A few required the senior boys to wear them as well, but many of these requirements were generally dropped in the 1970s.
The 1920s: Short pants became commonly worn at preparatory schools. At the same time blazers and shirts with soft collars replaced Eton suits at private schools. I'm not sure to what extent state schools required shorts. I think it may have been more a reflection of what boys were wearing at home. The private schools, however, tended to have very detailed uniform requirements.
The 1930s: Shorts were almost universally worn at English elementary schools and preparatory schools. I do not think that regulations at the state schools required them, but the private preparatory schools probably did.
The 1940s: I do not know of any major change during the 1940s. Some unconfirmed reports suggest that war time shortages incouraged the production of shorts rather than long trousers for boys, but I am not sure about this.

Figure 4.--The shorter Continental style of short pants became common at British schools in the 1970s.

The 1950s: Short trousers were very commonly worn by English schoolboys. I think it was in the 1950s that formal uniforms became more common at state secondary schools. Many began required short pants for the younger boys. State elementary schools rarely required uniforms but many elementary-age boys normally wore shorts. The shorts commonly worn continue to be knee-length often baggy shorts.
The 1960s: Many state secondary schools in the 1960s began relaxing uniform requirements. Many dropped the requirement that the younger boys wear shorts. State elementary schools, however, began to introduce school uniforms and many began requiring the boys to wear short pants and kneesocks. Continental styling with shorter more trim its appeared. Almost all prep school boys continued to wear shorts.
The 1970s: Prep schools where all boys had once worn short pants began to drop the requiement or permitted older boys to wear longs. Many elementary schools also began to drop the requirement for boys to wear shorts. Many dropped the uniform requirement all together.
The 1980s: The number of state elementary schools requiring shorts continued to decline. They were still commonly worn at prep schools, but increasingly as the decade progressed by the younger boys.
The 1990s: School shorts became less common in the 1990s, even at very traditiinal prep schools. Often shorts were made seasonal summer wear and during the colder months boys were permitted to wear longs.
The 2000s: Trends for the new century are not yet apparent. It is likely, however, that many schools still requiring shorts will make them seasonal; summer wear rather than year round wear. Schools are also likely to make the formal dress uiform a blazer worn with long trousers.

Schools had different regulations involving short pants. Common practices differed widely from school to school. Many elementry and prep schools were all shorts until the 1970s when older boys were allowed to wear longs and various regulations were instituted to govern this.
Age: Some schools set an age limits for wearing short pants. A common regulation at prep sschools in the 1980s, for example, was to allow boys to wear longs when they reached 11 or 12 years of age, depending on the school.
Form: Another common requirement was to base the wearing of shorts on the form (grade) the boy was in. Usually boys in the 5th or 6th form of prep schools were allowed to wear longs. This was the general practice at the private and state secondary schools which continued to require short pants. Usually it was the first or second forms that had to wear them.
Height: Less common, but used at some schools, wearing shorts was determined by height. This was less popular than other practices because it meant that if a boy was short, he might have to wear shorts, even as a senior boy. It was mostly used at a few public (private secondary schools). Sedbury in Yorkshire, for example, in the 1960s set a height of 5 feet 6 inches to qualify for wearing long trousers.
Optional: Some schools made shorts optional. In such instances at most schools the boys, except for the vary youngest would usually wear long pants.