My unscientific survey of summer students was that
they either came
from scouts, or were people working on advanced degrees in college.
Not all high-school summer employees were scouts (or scout equivalents -
kids who had logins on BTL Unix machines). I think in particular of Steve
Johnson and Stu Feldman, who eventually became valued permanent employees.
The labs also hired undergrad summer employees. I was one.
Even high-school employees could make lasting contributions. I am
indebted to Steve for a technique he conceived during his first summer
assignment: using macro definitions as if they were units of associative
memory. This view of macros stimulated previously undreamed-of uses.
Doug