There are only two conferences in all of DI that I know of with an Academic Index ('AI'); the Ivy League and the closely related Patriot League.
So I doubt that the Big Ten has any standards defined for football recruits and if there is, I doubt they're higher than the NCAA guidelines. Prove me wrong, I'll gladly admit it.
As to the individual schools in the Big Ten (or any individual school in DI, for that matter), yes of course they can choose not to admit a football recruit because he doesn't meet the school's minimum standards. Every school can always choose to do that, if they see that the minimum NCAA standard is not good enough.
I also recognize that the U probably has higher than average admittance standards for any new undergrad.
That said - I flat out don't believe that the U has ever or will ever deny admission to a football recruit, so long as he meets the NCAA standards. Why would a school, especially a public school, ever do it? It does literally nothing to the overall student population to admit a few "low scorers". No one cares how these guys do in school - so long as they score points and make tackles. Prove me wrong. Provide me a documented case where the University of Minn denied admission to a football recruit that the coach wanted because he met the NCAA minimum academic standard, but that wasn't good enough for the U.