Take the Blue Line to the Airport station. These shuttles are free and run about every 3 minutes. There are several different Green Line trolleys that stop here, so be sure you get one marked Boston College. This car will take you to the Boston College terminal, which is the end of the line and just across the street and slightly downhill from your on-campus housing. Plan on about 1 hour from South Station to Boston College.
South Station is two stops down the line.
Allow about 1 hour from Boston College to South Station.