Senior Prom by Rosamond du Jardin (1957)
A New Year's Eve spent with Steve Judson convinces Marcy that from now on, she and Steve will be "just good friends."
Luckily, her last term in high school provides many distractions: work on the school paper; the effort to get permission for a senior trip to Washington; a growing friendship with reliable Rick Whitney; and a date in prospect with exciting Bruce Douglas for the long-awaited Senior Prom.
The Marcy discovers that Bruce's plans for that gala event are not at all what she has in mind, and is she doesn't go with him, she can't go with Rick, because he's out dating someone else--at Marcy's unselfish insistence! (from the back cover)