For the school year of 2013-2014, according to school calendar, we get the following numbers:

School year started one day after the Labour Day in September.  The last day of classes was June 13.

From the Labour Day to June 13, there are totally 41 weeks.

Winter break: Dec 23 - Jan 3; (2 weeks)
Spring break: Mar 10 - 21; (2 weeks)

Actual weeks of classes: 41 - 2 - 2 = 37.

Statutory Holidays: 7 days (including Labour Day, excluding days in winter break)
Pro-D Days: 6 days (Sept, Oct, Nov, Jan, Feb, May)

37 weeks x 5 days/week = 185
185 - 7 - 6 = 172.

Actual school days during a whole school year: 172 days

Note: During these 172 days, there are still many days that have no class.  For example, there was no class on Sep 3 when the school opened.  Another example is those "study hall" days.