Well said Hazir. Incidentally if you follow the logic of opposing a one-size-fits-all approach to its logical conclusion then it is totally meaningless and irrelevant to come up with a "fair and reliable" way of measuring performance. There is no objective fair and reliable way to measure my industry either but who cares, since the state isn't involved it isn't necessary to come up with one either. I either impress my customers (with whatever varied matrix of unique desires they have) or I don't.

So impress your customers. In this case, if the funding largely follows the pupil, that would be the parents. No need to come up with a fair and reliable metric, just do that. The state can then have abstract and potentially unfair targets that suit political interests - eg greatly improving standards in a poorly performing area, being a centre of excellence, taking on the toughest pupils and so on and so forth for bonuses. If they're not fair, then boo hoo, get over yourself.

If the school is freed from the shackles of a bureaucratic state then it too has no need to measure "fair and reliable" objective performance matrices. Free the Head Master/Governors etc to have more flexibility with the funding and provide reward funding if appropriate to whomever they deem worthy. Based on whatever metrics they deem worthy. Are you that bureaucratic that you think no colleague ever deserves a reward unless all can get it because it may not be objectively fair?

Life's not fair and if your attitude is that we can't do anything at all because it can't be objectively fair and reliable then that's a terrible attitude to pass on to kids. We do the best we can with the tools we have.