I would probably make a whole sheet cake instead of 1/2 sheet or half sheet chocolate and half sheet white with buttercream icing.  Unless you are cutting the pieces in 1X2 inch squares, you are not going to get 40 servings out of a half sheet.  A half sheet is supposed to yield 18 (2"X2")-40 (1"X2") pieces, but remember you are looking at college kids.  A whole sheet cake is supposed to feed about 48-90 people, but I have always found that it's better to plan for more cake than to run short.  I like to calculate for the lower number and usually only have a small amount of cake left over.