The Harvard Summer School is a throwback to an earlier era. Prior to World War II, undergraduate admissions requirements had not yet reached the intensely competitive level it took on during Harvard's post-war democratization and transformation into a semi-meritocracy.
In those days, only a small financial aid program existed, and ability to pay was [almost] as important a criterion for admission as intellectual ability. As a result, Harvard was mainly an enclave of the upper-middle and middle classes, who sent their sons--and occasionally their daughters--here for a diverting four-year waystop on the road to success.
As with the Harvard of yesteryear, the Summer School's academic requirements for admissions are fairly loose. In fact, they're virtually non-existent: all you need to get in is proof that you're enrolled in a degree-granting institution of higher-education, or, if you're in high school, a letter from your guidance. The chief criterion for admission is ability to pay, and as there's no general financial aid program, most of the students here are people whose families are well enough off that they can avoid the tedium of a boring summer job, or go-getters who are attending classes and working on the side.
The result of this "open admissions" policy is that the Summer School community is a diverse one, ranging from high school students looking for a taste of the big time, pre-professional trying to get a head start on requirements, and college and graduate students looking for a "meaningful" summer.