[Qwertious]

That's not as relevant as you'd think - the "customers" of teachers are, arguably, taxpayers and not children. Alternatively it's parents, or perhaps politicians who set the budget.

Focusing on children is the more pro-social preference, but children who attend schools famously don't have jobs in functioning societies.

More importantly, the teaching system is basically un-incentivized by incentives - teachers in the US (focusing just on the US for a sec) are incredibly underpaid, and basically rely on masters-degree teachers putting in effort completely disproportionate to the pay. Everyone accepts this state of affairs specifically because we're all ignoring market incentives in favor of the good of society (i.e. the quality of childrens' education).

So, let's suppose we judge teachers based on how well they appease politicians and parents who support their funding: they pass all students, regardless of how poorly they do on tests and how badly the children need to just repeat the year. This is a terrible outcome, and yet you're implicitly endorsing it.