The pictures drawn on the blackboard change frequently.
Kevin and Paul are in Kevin's bedroom, reading the sex ed book they'd received earlier that day. School had only recently started, so the timing of the episode was likely September. On the wall behind Kevin is the 4 photos of The Beatles that was included in the packaging for "The White Album," which was not released until November 22, 1968.
Which takes place in September 1968, Kevin and Paul steal a copy of the book "Everything You Always Wanted To Know About Sex: But Were Afraid To Ask," by David Reuben, M.D. from the bookstore. However, the book wasn't published until June 1969.
The photo inserts from "The Beatles" (aka "The White Album") are shown hanging on Kevin's bedroom wall while he and Paul are reading through their textbook. This episode takes place in September, 1968, but the album wasn't released until November.