Honestly, she had no idea there was a party going on (or maybe she had an idea, but didn't care enough to remember) nor did she have any intention of attending said party. She didn't know these students well enough to wish any of them well, so why bother?

So when she accidentally walked into the wrong classroom (having forgotten a book in one earlier in the day) she was very surprised to see this going on. Maybe she could come back later...