The problem with writing "Sep 2011 - Aug 2011 = studying" is that, when the officer cross-references with your transcripts and does not find summer classes(i.e missing information), he/she will return the entire application.
I'm pretty sure that CIC sees a lot of variation in how this history is completed, so don't worry too much if someone has done it differently than you! But want to comment that summer classes are NOT expected, the typical student will have fall and spring classes - it will not be a problem to not have summer classes.
I listed my undergraduate time in a 4-year block (e.g. August 1990-May 1994) as that was my main activity for those four years.
But if I held a summer job elsewhere, I listed that separately (June 1993-August 1993). I called CIC and they confirmed it was OK to have overlapping time frames - just NO GAPS!