On the Thanksgiving episode, Lauren (the teacher) is inviting everyone to her house for Thanksgiving. Well, Thanksgiving is a national holiday and school is let out for it. So, if it really was Thanksgiving, school would have not been in session.
And what about the fact that it always seems to be dark when the teachers leave the building?! One of the perks of being a teacher is you can leave work and still have a lot of your day to enjoy!
It depends on where you live, sometimes the sun sets earlier than in the places you are used to. Also, many people celebrate holidays on a day that everyone can come, not on the actual holiday.