The kid in the Math class which was wearing glasses has a picture in Cosmo and Wanda's castle in the Hall of Infamy. This means that he could have been the last god child of Cosmo and Wanda before Timmy became their god child.
This episode was ranked #38 in the 100 Greatest Nicktoon Episodes countdown.
Apparently, Mr. Crocker keeps some fairy hunting gear in the janitor's closet.
Apparently when it is the last day of school, Francis beats up every single male student in the school for his own pleasure.
Early airings featuring Wanda and Cosmo in Spanish class had the teacher saying in Spanish (and written on the chalkboard) "Where is The Government Cheese?". Afterwards, Cosmo presents a wedge of cheese with a USA label on it. For reasons unknown (probably due to the joke being unintentionally politically incorrect), later airings featured this scene with the phrase (spoken and written on the chalkboard) ""Where is the smelly cheese?" with Cosmo presenting a wedge of cheese with no USA label this time. This version still airs on TV today.
The man that joked about the clapping invisible hands has a weird cat-like face.
The title is a pun on the word "invisible".
Albert Einstein - In Timmy's school, there is a teacher that bears great resemblance to him.
Timmy doing mischievous stuff while he is invisible.
Timmy's parents always being in their car, even when inside a building.
When Francis stands up towards Timmy's desk, Mr. Crocker doesn't seem to mind that he is disturbing the class.
Elmer is drawn differently in this episode, most notably without his boil. Although it could be that it is a different kid.
In the beginning of the episode there was a metal fence on the front of the school like it was a private school. But in most episodes the metal fence is gone.
After the close-up of Mrs. Turner talking inside the car in the school, you can see Cosmo and Wanda's Sombreros, but in the following scenes, they are nowhere to be seen.
In the original version with the Spanish teacher asking the question, "Where is the government cheese?", the teacher incorrectly refers the word "government" as "governmente". The Spanish word for "government" is "gobierno".
When Timmy wishes his fairies looked like school kids, Cosmo's long sleeve is missing.
When Timmy enters to the classroom where the math class is, there is no door on the entrance.
Also, when Timmy enters to the clasroom, the teacher was near the students' desks. But in the next scene, he is beside the chalkboard.
When Cosmo and Wanda were inside the car as balloons, in one scene Wanda' eyelashes are missing.
In this episode, A.J. is rumored to be a vegetarian, but in other episodes, he eats sandwiches with meat.
Possibly the rumor is incorrect rather than a goof.
A "Girl" (In a high voice): Hey there Francis, it's just me. Me, the girl. Not the boy you were going to pound if I were a boy which I'm not 'cause I'm a girl.
(Francis took off the "Girl"'s wig revealing she was a boy in disguise. The boy then switched back to his regular voice)
The boy: Please Francis no, I'm still wearing a dress.
Mrs. Turner: Anything else before we drop you off, don't look back, and pay absolutely no mind to that scary gray child?
Timmy: Look! Over there! A Rottweiler!
Francis: I'm not falling for that one, Turner.
Timmy: You're right. It's just a brick wall.
Francis: A brick wall? Where?
(when Francis turned his head, Timmy ran out into the hallway)
Wanda: Y'know running away from your problems never solved anything.
Cosmo: But it's great cardio! Run, Timmy! Run!
Timmy: Cool! You can see right through me!
Wanda: You're ten. You're not that hard to figure out.
Francis (to Cosmo): Well when you see him, tell him I'm gonna give him my special year end punch or I'll give it to you.
Cosmo: Oh cool, I love presents!
Timmy: Nobody does that to my friends as long as I can't be seen defending them!
(Francis hurt himself by hitting Timmy's medal four times)
Timmy: Well what d'you know, it doesn't hurt to have perfect attendance.
Mr. Turner: And look at that neat, dented medal you got! In my day, all we got were the cruddy, undented ones!
Mrs. Turner: Anything interesting in your last day of school?
Timmy: Hmm, not really. In fact, if you didn't know any better... It's like I wasn't even there.