Try Your Worst, by Chatham Greenfield

Sadie and Cleo have been locked in competition with each other literally from birth, when their mothers tried to produce the first birth of the year (Cleo won that competition, but Sadie was the last baby born in the previous year).  Growing up, both girls competed for awards and grades.  Now, they vie for salutatorian and valedictorian.  At times, the competition grew intense and the girls lost perspective.

However, their senior year seems to be out of control.  Lockers are getting vandalized, tires slashed, and precious belongings stolen.  Their principal calls them in to warn both girls that they need to tone it down or he will expel them, putting an end to the competition for good.  However, neither Cleo nor Sadie are actually doing these things.  In fact, it seems that someone is actually trying to get them in trouble.  That's not hard to do as their homophobic school principal does seem to have an axe to grind with them.  

The question is who is doing this and can the girls expose that person before they succeed in getting them thrown out of school?  Complicating the investigation is the nagging suspicion that each of them have that the culprit is really the other person.  Sadie suffers from clinical depression and her unpredictable behavior makes her especially suspect and vulnerable.  Predictably, the shared adversity of having a secret enemy brings the girls together into an unexpected romance. That makes things yet messier as their trust in each other is serious tested throughout.  A bittersweet ending eschews easy solutions and felt right, even if it was also terribly unfair to the protagonists.

The competing storylines (mysterious malefactor, coping with depression, and romance) leads to a confusing story that I eventually lost track of.  The depression story, in particular, didn't work for me and seemed largely unnecessary.  That said, I enjoyed the characters and their struggle and rooted for them.