"High school" (aka GCSE for our English viewers) electronics classes were almost 20 years ago for me and I studied mechanical engineering at Uni, so please don't think I actually know what I'm talking about when I make these requests
I was thinking that maybe you could connect a capacitor to the LED output and have it charge a little everytime that the LED's came on, and when the final hit came it discharged and triggered the cut out relay. The problem with that bright idea is that I suspect the wide range of hits allowed for the various classes means that the super light weight wouldn't fill the capacitor before it was supposed to be dead, and the Heavy class would trigger before it took all the hits it was supposed to?
(or is my recollection of those classes and how capacitors work getting a little too fuzzy? [] )