There is a relatively new anti-gun website called Armed Campuses that tracks which colleges allow guns on campus.
Currently, 25 colleges and universities in four states (Colorado, Utah, Michigan and Virginia) allow the concealed carry of weapons. The site features the map below.
The site writes:
The overwhelming majority of the 4,300 colleges and universities in the United States of America prohibit the carrying of firearms on their campuses. These gun-free policies have helped to make our postsecondary education institutions some of the safest places in the country. For example, a 2001 study by the U.S. Department of Education found that the overall homicide rate at postsecondary education institutions was 0.07 per 100,000 students in 1999. By comparison, the criminal homicide rate in the United States as a whole was 5.7 per 100,000 persons overall in 1999...
Despite the success of these gun-free policies, an increasingly extreme pro-gun movement in the United States has been active in advocating for legislation (and litigation) to force colleges and universities to allow guns on campus.