With luck, it should work in your favour if you are keen to step down on satisfactory terms and let the school regroup and move on under a new head with minimum fuss. Fingers crossed. Obviously, you need to be mindful of the risk that they would prefer instead to shunt you aside into some awful backwater job, but I’m sure you will have your answer ready on that.
Ideally (for you, anyway), the whose opinion carries most weight in the IHR decision will be responsible for your future salary costs but not your IHR costs! In that happy event, the negotiation might be short and satisfactory to all parties. But it is more likely, alas, that it won’t be that simple and that you will have to put your negotiating hat on and play a cool hand and maybe a longer game than you might choose. But these things do tend to work out in the end, and those scan results, grim as they are, should surely be powerfully persuasive. Good luck.