I am using Excel XP 2003.

Excel's subtotals feature lets you collapse rows and show summary data. It
displays little "plus" and "minus" buttons to show or hide the extra rows.

I'm sure I have seen a spreadsheet where this same technique has been
applied to columns, but I cannot find out how to do it.

I'm creating a spreadsheet where I have a bunch of data about pupils in a
school. At the end of each year they take 11 exams, and I would like to store
all the grades in separate columns for each subject, but I would like to
collapse them down to display an average for the year.

Is this possible?