When reffering to the school days, people often say
中学校の時. Even though this may not be wrong, they are shortened forms of relatively complex (redundant) structures. Thus, they are close to the expressions:
'when the school that I used to attend was an elementary school'
'when the school that I used to attend was a junior high school'
I think that there is a much simpler way to express the same situation, such as:
'when I was an elementary school student'
'when I was a junior high school student'
which would be shortened to
中学生の時, and I do hear people (including myself) using the latter. But still, many people seem to be using the former. Why do people use a more complicated way of saying it?