Number of US schools: http://nces.ed.gov/fastfacts/display.asp?id=84 refers to: U.S. Department of Education, National Center for Education Statistics. (2012). Digest of Education Statistics, 2011 (NCES 2012-001), Chapter 2.
Cost of Police Officers: http://bjs.ojp.usdoj.gov/index.cfm?ty=tp&tid=71#
Cost of new AR-15: http://www.atlanticfirearms.com/ar15-rifles/bushmaster.html
Number of Assault Weapons in US: http://www.slate.com/blogs/crime/2012/12/20/assault_rifle_stats_how_many_assault_rifles_are_there_in_america.html
Column one: ($116,500 fully loaded cost per officer) x (98,817 public schools + 33,366 private schools) = $15,399,319,500
Column two: (3,750,000 assault weapons) x ($1,000 for each weapon + $200 administration) = $4,500,000,000
Note: This graphic over estimates the cost of a gun buy back program and underestimates the cost of police in schools. Columbine had one officer present and one officer nearby during the shooting, which most observers would conclude was not sufficient to manage the situation. One officer per school would be less effective. The cost of the buy back assumes 3.75 million assault weapons; a presumably high estimate made by Slate.com, and the buy-back price is set at $1,200 per weapon (the price of a new, high end, AR-15) to allow $1,000 per weapon and $200 per weapon to administer a program. Finally, note that the police cost would recur each year.