It doesn't quite work that way under the current system because judgments about the quality of teaching and learning are not arrived at simply by aggregating lesson grades.
In earlier versions of inspection, when they were longer and involved much larger teams, inspectors would typically see dozens of lessons - perhaps 50 in a primary school and 100 or more in a typical secondary school. Often they would be full lessons. This would give quite a large sample from which it would be reasonable to cite percentages.
Guidance in those days was that if 10 per cent of lessons were unsatisfactory, that should raise the presumption the school was not providing an adequate education.
Things have changed. Inspectors now see a smaller sample of lessons. In order to test out the school's own view of the quality of teaching and learning, inspectors use direct lesson observations together with a range of other evaluation measures, such as scrutiny of pupils' work, an analysis of the outcomes for learners and discussions with managers and pupils. They will also look at school leaders' monitoring of teaching and learning and any external monitoring that has taken place.
Inspectors tend to see only parts of lessons because they are there principally to gauge what teaching is typically like in the school. The samples are not large enough to give statistical significance to the proportion of each grade: so quoting proportions for each grade could be unhelpful. If I am in a one-form entry primary school and I see a particular lesson that is judged to be ineffective, it would be wrong to extrapolate statistically that all teaching is inadequate and the school is therefore failing.
Inspectors would only reach that conclusion after they had seen similar weaknesses in several classes, and had other evidence of shortcomings, for example from pupils' work, that showed that pupils were not making enough progress.
It is also important to remember that inspectors select which lessons to observe for a variety of reasons and sometimes the proportion of inadequate lessons is not expected to reflect the overall quality of teaching.
That said, changes proposed to the inspection system over the next year or so could see an increased focus on direct observations of teaching and learning. This could presage a change to the way in which inspectors' lesson observations are used in whatever arrangements are finalised for the next round of inspections.
A rather larger sample of lesson observations could conceivably mean a return to guidance whereby a certain percentage of ineffective lessons automatically triggers consideration of special measures or a notice to improve.
Selwyn Ward has been an inspector for 15 years, working in primary and secondary schools. The views expressed here are his own. To ask him a question, email him at firstname.lastname@example.org.