Instead of using a lookup I would use a Stored Procedure.  Write a stored procedure that has an input of the exam number and returns only the dates that a greater than or equal to the current date.

Something like:

USE [databasename]
GO

SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO

CREATE PROCEDURE [dbo].[procname]
    @examnum int
AS
BEGIN
    SET NOCOUNT ON;

    SELECT [examdate]
    FROM tablename
    WHERE examnumber=@examnum AND examdate <= GETDATE()
END