I've only ever had to let 2 people go in 5 years of having employees, both college part time guys.
It needs to be made very clear up front your expectations and how your company runs. I do expect a little goofing off from someone in their late teens early twenties... wasn't too long ago I was there. However, when it's time to work, it's time to work...
If someone wants to fuss about the $10/hour part time I pay, I tell them to go to McD's and flip burgers for $7/hour and get less hours and less freedom.