Note that grade equivalents are kinda dumb things and not well-respected by psychometrics geeks. If a schoolie wants them and will listen to them, okay, I guess (I've put them in reports a few times for that purpose) but it's best to base arguments in better statistical grounding, or to give up on the statistics altogether and base the arguments in terms of how the kid's actual skills match up to the actual skills expectations of the actual curriculum.