Two teenagers meet and fall in love in high school (guess the title says it all). They might split up and meet again in later years or they may continue to face difficulties in their relationship.
These books will likely be more contemporary than any other, though books that have aspects of fantasy and the supernatural are not excluded. Books like Across the Universe by Beth Revis (a great book) and The Iron King by Julie Kagawa (also a fantastic novel) are not applicable to this book list because even though the characters are teenagers and in their high school years, they, technically, did not meet in school. Ash and Meghan, in the Iron Fey series (a fantastic series by the way), did not, actually, meet in high school even though Meghan was going to high school at the time. Also, Ash was not a "member" of high school when he met Meghan. With that said, paranormal books or series like the Hush Hush Saga by Becca Fitzpatrick are permissible, because both characters were members of high school and fell in love while going there. The term "high school sweethearts" doesn't apply to books where the student loves the teacher like in the case of Slammed (also a fantastic book) by Colleen Hoover.