The Textbook - Phan
It's my first fanfic, I'm sorry in advance.
Basically a high school au where Dan and Phil are in different French classes, and the school has a limited budget, so they have to share textbooks with someone in another class. Surprise (you guessed it) Dan and Phil end up sharing, and Phil leaves himself little notes in the textbook, which Dan finds.
You have to read to find out the rest, mofos.