This is a difficult question. My quick and fast answer is to say that the average PA student could probably get into med school. Unfortunately, its like comparing apples and oranges. Med school pre-reqs are usually much less stringent, with the only real science pre-reqs being bio, gen chem, org, and physics. None that I know of require A&P, biochem, or any other upper level course. But, the vast majority of med students has a degree in some biomedical science, meaning that most have actually taken the same pre-reqs that PA schools require...to include A&P, nutrition, biochem, genetics, gen physio, etc...
My belief is that unless someone applying for PA school has taken the MCAT, then you can never know if they would get into med school. I think a lot of people that are intimidated by the MCAT these days avoid it and apply for PA instead. Hence the young generation of PA's. But I think you can safely say that if you have the average GPA for med school (3.7 or so in both science and overall), an MCAT 28 or above, and good interview skills, that most will get into med school. Its just honest to goodness that MCAT that is so brutal and is really the weeder out!!