So as far as I know...and this could be poorly explained...
College teams have to have a certain amount of their players on track to graduate. The NCAA created the APR system, which takes into account the players who are academically eligible or those who leave while ineligible. I'm not sure exactly what happens once a player leaves after one year. I would assume, that as long as the player had been progressing towards a degree during the year, and leaves in good standing, then there's no penalty to the team's APR score. To my knowledge, only a couple players have ever had issues during their single year.
And on a side note, UK has had a few players who stayed multiple years and earned degrees along with some of the very notable one-and-dones returning to work toward degrees.
It's a broken system.