This all sounds very familiar... it also has to do with your status as a TA, I think. Since you're new, you're never entirely sure of the events at school, but no one else thinks to tell you about them, since they've done it all before. From what I recall, school staff are all so engrossed in what they have to do (read: complaining about their job) that they forget to explain things to the newbie. Being a TA and low on the totem pole doesn't really help either.

At least you got to attend the party! I guarantee at the end of the year, they will all be so impressed with your work ethic and that you participated so much in school events.