Factual error: There is a poster of Shaun Cassidy hanging on Susie's bedroom door - not only was Sean Cassidy only 15 years old at the time, but he hadn't even had a hit single yet.
Suggested correction: If you're talking about the poster visible at the 16-minute mark, I'm pretty sure that is Shaun's half-brother David Cassidy who was at the height of his popularity in 1973.
It's definitely David Cassidy.