Saltwater moons by Julie Gittus

Lothian 2008. ISBN 9780734410290
(Age 15+) Invited to spend the weekend at a shack owned by his parents, Sun agrees without hesitation, as she can see that Tycho is as interested in her as she is in him. But despite the sentimental cover, making this appear to be a schmaltzy romance, this book concerns the growing realization of Sun's betrayal by Tycho's friend, Mark. At the party Tycho is diverted by his former girlfriend, and the drunken Sun goes down to the beach with Mark. Here they have fumbling sex, Sun for the first time.
Sun is in year 12, and so feels she is in control of her life and where she is headed, but her decision to be with Mark, changes her view of life. Her relationship with Mark causes incredible friction between Sun and her best friend, and when Sun stays away from school through illness, on her return the spite towards her has consumed most of the year group. She turns more and more to Mark, despite the interest from Tycho still being there, and so she must resolve what is happening in her life. Most realistic, the behaviour and language of senior students is most real. Senior students will relish this story.
Fran Knight