This problem amounts to a $2\times 2$ contingency table (actually 2 such tables if you look at pre- and post-intervention).  The variable for the columns could be the pass/fail counts, and the variable for the rows could be the treatment condition (control & intervention).

Assuming appropriate conditions have been met, this is a chi-square analysis, and the conventional effect size here is the phi-coefficient:
$$\phi = \sqrt{\frac{\chi^2}{N}}$$
(Curiously, this also happens to be the correlation if you code the two variables as ones and zeros.)

The convention with this effect size is to classify with the cut-offs of 0.1, 0.3, and 0.5, for small, moderate, and large, respectively.