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.
In every scene Mr and Mrs Turner are in, they are always in the car.
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, 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 opening title card is a reference to the classic book and subsequent film The Invisible Man.
Timmy's invisibility looks exactly the same as Dexter's from Dexter's Laboratory episode Surprise. This is not a coincidence since Butch Hartman work on this show as a writer before The Fairly OddParents.
When Crocker sees the floating frog kicking Francis, he refers it as a "floating frog" which is a reference to the real animal, The Round-tongued floating frog.
Timmy doing mischievous stuff while he is invisible.
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. 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.
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.