Complex sentence: It is the only high school in the world to still participate in "potato break", in which students start school two weeks before Labour Day to compensate for a two week break at the end of September and the beginning of October.
Split into simpler sentences: It is the only high school in the world to still participate in potato break. Students start school two weeks before labour day to compensate for the two weeks they miss at the end of September and the beginning of October.