Filtering on gathered statistics adds dummy empty rows and fails anyway

I have a simple data set:

Pupil A Teacher A   pass
Pupil B Teacher A   fail
Pupil B Teacher B   pass

And want to compute the following statistics:

Teacher     pass rate
Teacher A    50%
Teacher B   100%

The way I went about it is to create two columns with formulas, =unique(B:B) and =COUNTIFS(C:C,"pass", B:B, E2) / COUNTIF(B:B, E2).

Now, this works correctly but when creating a filter and ordering by pass rate, I got this:

Two-part question:

1. What am I missing? How to fix it?
2. Is there a smarter and a better way to do this?