Try:

Create Function Get_XY_Coordinates (@i_Pupil_Number int)

Returns int

Begin

Declare @iXYCoord int

SELECT @iXYCoord = sab.address_id

From sps_address_bridge sab inner join sps_stud_assignment_address saa on sab.street_number = saa.street_number

And sab.full_street_name = saa.street_name

And sab.postal_code = saa.postal_code

Where saa.pupil_number = i_pupil_number

Order by CASE WHEN sab.modifier IS NULL THEN '0' ELSE 'Z' END, sab.x_coordinate, sab.y_coordinate

Return @iXYCoord

END