The chief inspector awarded himself the judgement, which is not available to schools, in his final phone-in on radio station LBC this morning.
Sir Michael initially declined to give himself a grade following five years in the post, but when pushed by host Nick Ferrari, he said he would write “done my best” on his end-of-year report.
Responding to a mother who wanted the subject to be made compulsory because of the increase in sexting among young people, he said: “I think we have neglected PSHE over the last few years, and we need to reprioritise it in the curriculum.”
Currently free schools and academies do not have to teach sex education or PSHE, but education secretary Justine Greening indicated at the weekend that the government was looking at making them compulsory in all schools.
'Sex education must be taught by specialists'
Sir Michael said: “The big problem that inspectors find is that it’s taught badly in schools. Often it’s an adjunct, it’s something that’s bolted on to the school’s curriculum. It’s taught by people who are not specialists in teaching this important area of the school's curriculum and it doesn’t work.
“If it’s going to be made compulsory then it’s got to be a discrete subject taught by specialist staff. We have not got enough of those sort of teachers, and it’s always difficult to get really good people to teach sex education, British values and parenting and so forth.
“It’s a tough subject to teach well, particularly with older pupils who need specialist staff to do that.”
He also said that England was “flatlining”, in response to this month’s Programme for International Student Assessment results, but highlighted its better performance than Scotland, Wales and Northern Ireland.
Sir Michael added that the other home nations had not embraced reform in the same way as England, but acknowledged that the success of London had lifted England overall. He said that migrant children in the capital, whose parents were ambitious for them, were responsible for “a lot” of this, and that English parents had to do the same.
“The family is the great educator. It’s the family that provides children with the support they need,” he said.
Sir Michael later told BBC Radio 5 Live that Brexit could be bad for English schools if it led to fewer immigrant children.
He said: "Less immigration might mean actually lower standards because immigrant families want their children to do very well. They want a better education system than the one they experienced in their home country.
"That's one issue. And I know schools recruit teachers from abroad, as I did when I was a head. That could be a challenge in the future."