Lacy's Lane (The Lacy Trilogy #1)
Patricia Strefling

When her younger sister, Allison made the announcement Lacy felt her stomach twirl. Thad Gannon had been her high school crush, until he dumped her for another girl at her first prom. She had never fully forgiven him for that, nor the fact that he had abandoned his grandfather who lived next door.
Seeing him would mean digging up the past and that was not going to happen. She had buried some memories long ago deep inside and worked hard every day to keep them that way.
When he arrived, Lacy was going to give him a piece of her mind . . . if she could even look him in the eye after what happened that night after the prom.