That's balanced by the fact that the certificate of learning will be valued less. If that doesn't happen, you should reconsider whether people are actually purchasing an education, even if that's what it looks like.
Edit: in my example the university could change from accepting all students with correct diploma to doing admission tests to resolve. I know nothing of the feasibility of this though.
Then people will choose their high school depending on whether they need that extra preparation, and those schools will have more students or be able to charge more.