Andrew Furco from the University of Minnesota reviewed service-learning literature a few years ago. His report, "Growing to Greatness" indicates that service-learning has a positive impact on school attendance and increases prosocial behaviors, which may lead to fewer suspensions and/or expulsions. In the very important effort to narrow the achievement gap, this article in the Journal of Experiential Education lays out the positive impact of service-learning on attendance.
The service-learning dropout prevention research agenda is still pretty new so there isn’t a ton of evidence for it – just a recognition of the potential. Check out "Engaged for Success: Service-Learning as a Tool for High School Dropout Prevention" published by The National Conference on Citizenship in 2008.