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.