Just after his mom tells Forrest he's no different from anyone else, which only Forrest would believe, we cut to Forrest's school principal, who says Forrest can't attend a regular school because his IQ is only 75. The state requires a minimum IQ of 80.
Forrest's mom isn't going to give up on him. She wants him to have the same education as anyone else and is willing to do just about anything.
The principal then asks Mrs. Gump if there's a Mr. Gump in the picture. Turns out he's "on vacation."
Cut to Forrest, who sits outside his house while his mother has sex with the principal in order to get Forrest into public school.
Good parenting, or best parenting? You decide.
Now, we see Forrest's mother reading to him, and Forrest asks her what vacation means, like whatever his daddy is doing. She answers that vacation means when someone goes away and never comes back.