Night Before the Exam is about the difficulty of learning 4 syllable Japanese idioms called Yojijukugo. They are written in kanji and part of the dance shows AG trying to write them. The song's repetitive 4/4 beat echos the 4 syllables and emphasizes the frustration along with AG tearing at their hair and destroying the class room.
As you probably got by the end of it, they aren't partying, they ARE studying, and it's not going well at all. The red ribbons they use in this number are called "hachimaki", and they are stereotypical symbols of intense studying. Manga will often show a student with a hachimaki tied around their head as they furiously try to cram for a test. I think Mizyu gets extra credit in this performance, as she gets tangled up, falling over, inside the desk. She really goes all out.
