How should they divide the money so that each class gets the same fraction of the prize money as the fraction of the box tops that they collected?

Figure the fraction of box tops each class collected, then multiply the prize money by that fraction.
Total box tops = 3760 +2301 +1855 = 7916
Mr Coronado's class's fraction: 3760/7916 × $600 = $284.99
Mrs De Souza's class's fraction: 2301/7916 × $600 = $174.41
Mr Nost's class's fraction: 1855/7916 × $600 = $140.60