does anyone of the U.S. board members know what a "summer student" is?
The student here is an university student.
Does it mean that such a student takes part in certain classes over the summer vacation or that he simply works for some faculty over that period of time?
Yes, summer school was my first thought, too. Maybe it is.
However, in the book I'm reading, the author (an American) writes the following:
I discovered our accounting software to be a patchwork of programs written by long-departed summer students.
Come to think of it, it might have been a summer school project to write parts of the accounting software.
Well, at this point I think it is most reasonable to assume that a summer student is just a student who is attending summer school.
By the way, are they also called 'summer students' in the UK?
We have summer courses, so the term is logical.