I cannot speak for you as a student, but I can speak to my previous self as an 11th grade student.

The subjective probability of my passing a test is 1. In high school I never failed any tests, so the number of passed tests divided by the number of tests taken is 1.

If your experience has been different, you might make a different calculation. If you pass roughly 90% of your tests, then the probability that you will pass a random test will be approximately .9

Since this is a subjective probability you need not provide the exact number of tests. You can get by on decent approximations.