Take the T inbound to Government Center. Take to the end of the line. Boston College is across the street. A shuttle bus will take you to the appropriate airline terminal. The rates will vary.
Some agencies also rent vans which hold up to twelve persons.
The Green Line has four branches, three of which have stops within walking distance of the Main Campus. This is the slowest of the three lines serving Boston College. This branch is the fastest. These may be purchased at the Park St., Harvard, Government Center and Back Bay Stations and the North and South Stations.