The Class Year field in Raiser's Edge can be formatted to display and export as a 2 digit year or a 4 digit year. If you have the Raiser's Edge options set for the 2 digit year, when this field is exported, it comes out as a string field, which sorts in Crystal Reports in alphanumeric order. This order does not sort the records in the same way as they would if they were treating them as a year - for instance, 2000 would export as '00 and 1999 would export as '99. This sort order would put '99 as occurring after '00, whereas when sorting as a date, you would want 1999 to come before 2000.
In order to get the sort to work correctly, you will need the full 4 digit year. While it is possible to create a formula to tack on the 19 or 20 prefix, Crystal Reports isn't going to know whether the original class year should get the 19 or the 20. For instance, a Class Year could export as '12. Crystal Reports is not going to know whether the Class Year should be 1912 or 2012. To ensure the most accurate results, you will want to change your Raiser's Edge settings to use the 4 digit year instead.