I think it is "Sixteen Candles" by Nicholas Pine. Published 1994 in the Terror Academy series.
Synopsis
Once she was a quiet wallflower. Now about to turn 16, Kelly has slimmed down, shaped up and become one of Central Academy's most popular students. She's put her parents' mysterious deaths behind her and come out of her shell. But a killer still stalks her, and her next date may be her last.
