My school (University of Arizona) has a pretty detailed policy. Basically 1 D or F gets you a warning and you have to repeat the course. A second one gets you probation and a repeat. A third one gets you kicked out of the program. If you fail the class again, you are kicked out. Failing a class will pretty much guarantee an automatic extra year in school because everything builds on itself. I think there is a bit of leniency though if you are asking for help and seriously putting the effort in. All of my professors want to see us succeed, so I feel this policy is a fail-safe to kick people out who aren't trying.