Looking for Alaska is definitely fiction. It's a work of the author's imagination, filled with characters and events that are not based on real-life happenings. The plot and settings are crafted to tell a compelling fictional tale.
You can try searching on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfics, and you can filter by the 'Rated M' tag. Just use the search bar and type in 'Looking for Alaska' to find relevant M - rated fanfics.
Yes, it is. John Green incorporated elements from his time in boarding school into the story. It has that sense of authenticity as it reflects on real - life situations like friendship, love, and the search for meaning that he might have witnessed or experienced himself during those years.