Even though the student may be "away at school", she is, normally, considered as still living with her parents, for tax purposes. As such, the cost of that home is included in the support test.
The support value of the home, provided by the parent, is the fair market rental value of the home plus utilities & other expenses divided by the number of occupants. Other support expenses include clothing entertainment and transportation.
The IRS has a worksheet that can be used to help with the support calculation. See: http://apps.irs.gov/app/vita/content/globalmedia/teacher/worksheet_for_determining_support_4012.pdf
