That's the precise reason. That's also why schools don't give off for major (but less observed) Jewish holidays like Sukkoth, Pesach, and Shavouth. It's not the religious significane of the day-- but the prevelance of it's observance. Those same teachers who would skip on Yom Kippur wouldn't skip class for Pesach.
Old joke, but maybe not so funny:
One thousand years ago somewhere in Europe a rabbi has a vision in his sleep. An angel appears in his dream and says "There is a continent across the Atlantic Ocean, and one thousand years from now there will be Jews there and this is how they will celebrate Chanukah." And the angel shows him.
When the rabbi wakes up he says to himself, "If this is how they celebrate Chanukah, just imagine how they must celebrate Shavous!"