Pascal, Francine. Sweet Valley High #1: Double Love. 1983/2008
They have decided to bring back Sweet Valley High, which is a series *I* loved when I was in middle and high school. They are basically re-packaging the series, bringing it up to date with cell phones, different cars, etc., but the plot lines remain the same. I don't know how many they plan to bring back-- they used to have a new book out every month. I find that SVH is basically a more wholesome version of Gossip Girl, so for schools that can't have them due to content, this is a good alternative.
Liz and Jessica are twins. Liz is the smart nice one, the one on the school newspaper. Jessica is the one that isn't so nice, the cheerleader etc. She has no conscience whatsoever. This first book basically introduces everyone to the case of characters that we will deal with in the rest of the series, but also focuses on the girls wanting the same guy, Todd. Jessica knows Liz and Todd like each other but she stops at nothing to get him.