Elite private schools have the highest graduation rates - Princeton, Duke, Georgetown, etc. are all 90%. There are probably several reasons why - smart/driven/organized students, students aren't part time, highly involved parents, good advising, ability to get needed classes, etc.
I would venture "not having a job while you are in school" is very important for this metric. Once upon a time, working more than 8 hours per week during college was the singular defining metric for graduating on time.