In Monsters U it's set up as if Mike and Sulley didn't know each other before attending MU.
But at the beginning of MI Mike says this to Sulley:
You've been jealous of my good looks since the fourth grade, pal.
This was a jest, but I want to know if the "fourth grade" comment was purely a joke as well.
Did they know each other in any way before MU?
Any officially licensed media will be accepted. Books, shorts, web content. If in has the Disney logo on it (legally) then it's valid.
Canonically, it was decided to be a joke, but it's in a deleted scene on the Blu-ray copy of Monsters University.
.... It was initially a problem for director and co-writer Dan Scanlon, who was determined to resolve the two storylines. “As much as we could, we wanted to be totally respectful of the continuity of the first film,” says Scanlon. “Our first inclination was to try to make it work, to do versions where they became friends in fourth grade. Basically, after trying it a lot, we felt like it was robbing the audience of the experience of really watching a relationship grow. And we realized we were going through all these hoops for this one line that doesn’t affect the story in a negative way. It was really [Monsters, Inc. director] Pete Docter and John Lasseter who said, ‘It’s not as important as you think. You’re not hurting the movie to ignore that line.’”
....
But before the filmmakers decided that the fourth-grade zinger from Monsters, Inc. had to be ignored or simply written off as a flippant retort, playground scenes were written and artwork was commissioned....
