Francine Paula Pascal was an American author best known for her Sweet Valley series of young adult novels. Sweet Valley High, the backbone of the collection, was made into a television series, which led to several spin-offs, including The Unicorn Club and Sweet Valley University. Although most of these books were published in the 1980s and 1990s, they remained so popular that several titles were re-released decades later.
After Sweet Valley High came the Caitlin trilogy, followed a few more books; I think I bought most of them in the States, and read this sometime in 1987 to 89, I think. The Caitlin series were a bit more daring the SVH books, just a bit. Think PG-13.