He won’t release it because he probably barely passed. If records are released they will be scrubbed clean and show a straight A student. I’m willing to bet on it.
I'm more surprised he hasn't done it already, that said, in his autobiography, he does state that he wasn't a good student.
Yet, that never stops the media (and sadly, even many conservatives) from professing or believing that he is somehow "book smart" or think that he was some kind of good student or high achieving academic ....