So, if any bias, they should admit with counter-to-general-society biases, since those who satisfy the biases of the general society don’t need preparation.
(Unless the goal is to just appear to be preparing people for success while doing nothing, in which case having biases aligned with those of the general society is the ideal strategy: pick the people that were going to succeed no matter what, and claim credit for preparing them.)
I do think this is what lots of schools do. I mean, "the best people available" will be roughly identical with "the pool of people that contains the highest fraction of those who would succeed no matter what". And if your alumni are successful, then you claim credit. Why not? (I've occasionally seen people attempt to calculate "value added", but completely accounting for selection effects is difficult.)
And, by the way, just because someone would have succeeded no matter what doesn't mean you can't increase their success. It's plausible that some highly-driven highly-competent genius would have ended up becoming a top engineer going to school X, but would have co-founded a billion-dollar company going to school Y, due to some combination of getting a somewhat better education and meeting the right people. So the value proposition may be real.
Biasing the tests for "better student" sounds obvious, but it's still a bias to be to clear.