Used several times throughout the film, initially it is where the gang originates from before they traipse through Coleman’s (Jason Robards) yard and flowerbed.
It’s actually behind the location for Coleman’s house, which is also right across from New Hanover High School, the school they attended. Most of the cast walked down this alley at one point in the movie.
These guys probably didn’t know that Sandy (Laura Dern) and Jeffery (Kyle MacLachlan) attended this very same school a few years prior in the movie Blue Velvet. This school has been used multiple times for various productions. In this movie, they didn’t even try to hide the actual school name, New Hanover High School, as seen just above Maureen’s (Ria Pavia) funky sailor cap.
There were also shots taken from the rear of the school, along Princess Street, which is where they entered after crossing through Coleman’s yard. Coleman’s house is directly across the street from the school.
And for Lainie and Joel’s make out spot, it’s once again at the school, which fits the story, as Joel just waited for Lainie’s dance class to end before getting her alone and her rushing off on her bike from the school. This area is now a set of what I assume are air conditioning units, so the small park is now gone.
I’m assuming that all interior scenes involving the school are from this same location. For the Valentine Dance, the fast exit by Bobby and Lainie spurs Joel to give chase, but Dumas (Matt Adler) sends Joel airborne instead. This fuels the raging fire within the troubled and troublesome Joel to get even.