In the opening hitchhiking scene, Lindsay and Kim talk about reading Jack Kerouac in English class before the teacher assigns "On The Road" to them in the classroom.
When the guys are being picked for baseball teams, two girls are laughing at the last three guys to get picked. One the girls is holding a hurley, a stick used in the Irish game hurling, and not to be found on a US baseball field in the 1970s.
When Lindsay and Nick are in class, Lindsay passed his seat as he's twirling a drumstick and asks "how's it going?" As this plays out you can see the shirt he is wearing is Lucky Brand, a clothing company that wouldn't exist until almost a decade after this scene is set in.