(CAUTION: The events potrayed in this episode is an account of what I THINK happened. This may NOT have been what actually happened on the show. But since there hasn't been an thorough explaination, I thought I share my theory to give you ideas.)
It was early in the morning a couple years ago, and the quadruplets wake up happy that today's their birthday. They walk downstairs where Richard, 7-years old Fireball and Nicole wish them a happy birthday. They give them a chocolate cake.
Richard, Nicole and Fireball: Happy 4th birthday!
They then eat the cake and head off to school. There, as they're walking down the halls of Elmore Elementary everyone is wishing them a happy birthday. They then walk into their 1st kindergarden year (in which Miss Simian was teaching... What? I guess she also teached at kindergarden)
Gumball: Miss? Today's my and sisters' birthday.
Miss Simian: Sit in your seat!
Other kids: You should do something to celebrate the quadruplets' birthday.
Miss Simian: OK, I'll "celebrate" this by giving you... a surprise test! (evil laugh)
Now upset, Gumball decides to write a message on his test. He then writes on the test "GIVE US CAKE YOU OLD, STINKING, BABOON!", folds the paper into a paper airplane, and throws it to her desk. Upon the little note reaching the desk, Miss Simian scoops it, unfolds it, and reads it. She then yells at Gumball and sends hum to Principal Brown's office. 10 minutes, Gumball comes back with a note in his hand and hands it to Miss Simian. Upon reading it she rushes out of the classroom. Everyone in the room thinks she's going to come back with a cake from the cafeteria. But instead, she comes back and tells Gumball that he has detention after school today. But Gumball argues that he's getting his birthday present after school but she refuses to listen. He then realizes that he's going to need to break out. So after school, Gumball devises a olan to escape. He goes to the teacher's lounge and sees a medical form for Miss Simian on the counter. He reads it and finds out that Miss Simian is allergic to some ingridents found in Coca-Cola. He then looks ad finds a huge bottle of Coca-Cola in the refrigerator. He then pours the entire bottle into Miss Simian's afternoon coffee. He then reports to Miss Simian's room before she got suspicious. She then tells him that she needs to go to the teacher's lounge to get her coffee. She returns 5 minutes later with her thermos. She then takes a sip and she goes into an allergic reaction and collapses to the ground. Gumball then runs out of the school.
Nicole: Where have you been?
Gumball: Uh... I was helping someone find a lost item.
Nicole then believes him and they drive off.