suppose a teacher in a large secondary school was having it off with an underage pupil (as in recent telly drama - God, I love Sheridan Smith...). The headteacher would apologise on behalf of the school even though he couldn't have had anything to do with the offence nor even knew it was going on. Of course he'd apologise. But that wouldn't mean that it was in any way his fault,

If that was on school premises then the head would have some responsibility because of a failure to have systems in place to avoid putting staff/pupils into situations where this could happen.
If it was outside school premises then there could still be some responsibility depending on whether 'grooming' etc. was happening within the school.

Of course, your argument appears to exonerate Boris of any responsibility for any 'parties' of which he was unaware.