[uup]

The test was mandatory for all HS students to take, but I do think there is an element of selection bias. Those who took the test on the weekend may have been more invested in the result. They certainly dedicated at least an extra weekend's worth of time.

[sokoloff]

It was "mandatory", but still had only an 85% compliance rate. I would bet that almost none of the most academically inclined students were in that 15%.