Hello forums

Data i'm working with: I have attendance data for high school students, by class. I have a row for each student, by each class.  So if I have 8 classes, I will also have 8 rows, one for each class.  Along with class name, I have number of possible class days attended and another column for number of absences. 

Goal: I would like to establish the rate at which students attend each teacher’s class in comparison to their other classes.  For example, if all of the students who have Mr. Smith’s Geography class have an average overall attendance rate of 92% but Mr. Smith’s classes are attended at a rate of 88%, then students are 4% less likely to attend his class than the rest of their classes.

I have tried to accomplish this using the exclude condition, but to no avail.  Perhaps i'm doing that wrong.