When Amanda Spielman was appointed Ofsted chief inspector, there were many - including the Commons Education Select Committee - who were very vocal about their doubts. But with every annual report and speech, I’m more convinced that few understand the big issues facing it more than she does.

So it’s good to see her making robust comments on off-rolling; a problem that, until relatively recently, people barely even admitted existed officially. In fact, it is within working memory that few would even acknowledge that, when the inspector called, difficult children vanished for a few days, and kamikaze teachers were sent on training courses. It is refreshing to see something so shamefully hidden brought so forcefully into the light by someone so prominent. It is easy, as many have done, to point at successes and feel complacent, but far harder to point to the problems and then walk towards them.

Off-rolling - the illegal/unethical removing of students, most often to boost exam results - is a complete failure of the school’s most basic function as a public institution: to serve the community it belongs to. Hospitals don’t turn away patients because they’re too ill. They do their best, and deal with the situations with which they are presented as best they can.

Exclusion, as everyone is fond of saying, should be a last resort. And we shouldn’t forget that it can often be the best thing for a student in the long-term, as Spielman acknowledges. I speak to many pupil-referral unit leaders and, make no mistake, good alternative provision is exactly the specialist, boutique care many of these children need. But off-rolling has nothing to do with that. It is a detestable dereliction of duty that views children as data points on a scatter graph, valuable only as much as they serve the school figures. This corruption of education’s mission treats children as means to an end and should rightly be driven from our system.

In our haste to do good, a note of caution: we must also do no harm. Not all children leaving the system in Year 10 and 11 are off-rolled. Many leave because schools have come to the end of a very long line and can see nothing more to be done. In these cases, rightly, they need alternative provision. I think there is a tendency for some schools to hang on to students as long as they can. They mustn’t be punished for trying their best to do what they can. Many PRUs I speak to say they wish, in some ways, they could get the most troubled children earlier, when they can have more of an impact on their habits and needs and beliefs. KS4 is often too late to have a lasting impact.

And data will be key to understanding this; we need to be able to discern when schools have high exclusion rates because of demographics; because they serve highly transient communities; because they struggle to recruit staff owing to location or area desirability; because they are working within the most difficult of circumstances; or because they are gaming the system. The difference is crucial.

We need to deter off-rolling without deterring deserved exclusions or genuinely elective home-schooling moves. That will be the dragon to slay before any others. Spielman, more than any inspector before her, has shown an abiding interest in data and evidence, and I hope this will continue to be her guiding light.

And, while we have a long way to go before we can say with evidence that Ofsted does more good than harm to the school system, if anyone can turn the Death Star of Ofsted from a Doomsday Device to a surgeon’s scalpel - healing more than hurting - it’s Spielman. Let’s see what the new year brings.

Tom Bennett was a teacher in the East End of London for 10 years. He is now the government’s behaviour tsar, and the director and founder of ResearchED, a grassroots, teacher-led project that aims to make teachers research-literate