The churn rate is the actual number of students vs the drop off of students between year levels.
For example, say we're looking at Prep > Year 1 with the following sample data:
Capacity of Prep = 100
Capacity of Year 1 = 110
Actual enrolled students in Prep = 80
5 students did not progress from Prep to Year 1
To work out the churn rate, divide the 5 lost students by the 80 students you started off with, and multiply that by 100:
= 5/80 x 100