Specify a sort on your relationship between student and address tables so that the "current" address is either the first or last row in your portal. (You can choose ascending or descending order here.)
Then you can simply refer to the related record in your calculation if it sorts to be the first record or you can use the Last() function to reference it if you sort it to be the last record.
There is also a 'Current Address' field which is a yes no check box on the address portal to select which is the current address.
That's not the best method, because you might end up with several addresses checked as "current". It would be better to set a field in the student record with the AddressID of the selected address. Then you can use the same field to define another relationship - or use a filtered portal if you have version 11.