I think this is best explained using examples: If a pupil in my class is targeting a grade B, and they achieve an A* then they are two grades above where they were targeting, so VA for that pupil is +2. If another pupil is targeting a grade C and gets a grade C, then they have a VA of 0. Finally, if another pupil is targeting a grade B and gets a grade C then they have a VA of -1. If those three pupils made up a class of three then the VA for the whole class would be 2 + 0 - 1 = 1 overall. So for your class I presume that if the grades are fail, pass, merit and distinction(?) then if a pupil was expecting a fail (on official school target grades) and got a merit then the VA would be +2, or +1 if they got a pass. I will add the caveat that VA is usually only worked out for the final exam grades that the pupils achieve, but if you are wanting to show progression from a starting point and fight your corner then I see no reason why you can't calculate VA in a small time frame - just make sure that when you show anyone the figures you explain/note that it is calculated to show the VA directly by you in the time you taught them (obviously if you do have final grades for the pupils then this is not a problem).