Actually I am sure that the ranking system differs between schools. A few of the schools I have applied to said you are ranked against your part time class mates for your first year. After your second year and so on, you are ranked against the full time students that entered a year after you. This is only the case at some schools. If you are unsure, just call up.
I am sure all schools are somewhat different. But, of those that I called they ranked PT seperate from FT students.