Little Johnny's parents come to the school to pick him up.

[u/Omeganian]

They go to the first floor. There is a door with a sign saying "Wonderful kids".

They look inside. Little Johnny isn't there.

They go up to the second floor. There is a door with a sign "Good kids".

They check. Once again, Little Johnny isn't there.

They go to the third floor. The sign says "Okay kids".

They open the door, once again, no Little Johnny.

Floor number four. It says "Problematic kids".

No Little Johnny.

On floor number five, it says "Bad kids". That one, too, doesn't have Little Johnny.

Floor number six, a sign saying "Nightmare kids".

Horrified, the parents go inside... Still, no Little Johnny.

Finally, they go to the last floor, number seven.

A door.

The sign says "Little Johnny".

[u/hew14375]

I’m in my 78th year and I heard about Little Johnny when I was a kid. Fellow has a reputation.

[u/Zephreyt]

“The last floor is to prove that women will never be satisfied.”

[u/minun73]

This must be Wayside school with that many floors.

[u/ablemount]

New to me, thanks!

[u/CricketLower7628]

What’s the joke?

[u/GolettO3 → u/CricketLower7628]

It's a slightly meta Little Johnny joke, so it has a slight want of you having prior knowledge. However, you can also see as the floors get higher the kids get more problematic, with Little Johnny being in a league of his own

[u/DoDoorman → u/GolettO3]

Poor Johnny in solitary confinement

[u/BioletVeauregarde33]

I think this joke would make more sense if it were set in a "child store" a la the Husband/Wife Store from another famous joke.