The diary as a literary form is as old as the hills. It's incredibly flexible. Authors of all kinds have kept diaries: the common writer, the king and queen, the rag-picker, the poet, the murderer, the mortician, the child, the student, the teacher, the philosopher, the philanderer, the concubine, the critic, the curmudgeon, the cur. Their interests in keeping diaries have varied as much as their vocations and luck. One element alone is common to diaries: the entry. In the strictest sense of the word diary, it should be daily, a daily entry. Writers often while keeping records of their daily lives, thoughts, remembrances, and imaginings, have worked toward stories. That's what you'll be doing in this class.
You write five entries a week and print out one or two of them each week to be read in class; then, after the middle of the quarter you select, arrange, and revise entries to see how you can put some of them together into narratives. Your entries are the subject of the course, and besides writing your own you read everybody else's soon after they are handed out, preparing to discuss them in the next class. You write under pen names, so think of one before coming to class.
Please buy both required textbooks, The Life of Johnston Volumes 1 and 2, by Kate Johnston, before the term starts and have read Volume 1. It's short and fun and a good example of what a person your age can do with the diary as a form of literature.