You can do this with benchmarks and written evaluations. On the courses page, you will see a space to enter benchmarks (the little help bubble will explain how to do this).
Then, under "Course Benchmarks," you can enter the dates that your student achieves varying levels of proficiency.
If you would prefer not to enter any sort of proficiency levels, you can use "Written Assessments" to describe a student's progress in a simple text format.
Both Benchmarks and Written Assessments can then be printed out in a student's "Progress Report," which you can customize to include whichever information you prefer.