Each unit decides what they want to be 'passing' but you are required to get a Good/Low to be comissioned. My unit requires Sat/Med for fresh, Good/Lo for soph, Good/Hi for juniors and seniors. If you cant get these, you are not kicked out -- I think you have to fail two or three in a row to get the boot. They will, however, put on some sort of remedial physical training program.
Obviously, though, the higher your score, the better.