I am a doctor. A easy route to being in front of the GMC is to double dip. You cannot be earning from two jobs at the same time. If your job plan states it is NHS time then you can’t cover the private sector. You can argue that it wasn’t ‘unsafe’ and that ‘all the work was done’ but it doesn’t matter and you will be in big trouble.
Yes, Lucked, but surely you can see that during lockdown when schools were closed, if the school's only requirement is that its teachers set (and mark, where appropriate) a specified amount of work each day/week and make it available online for each of the classes they teach, then the teacher could be preparing and uploading those lessons at 8pm, or indeed 2 in the morning if they feel like it or if that's what suits them.
So, while tutoring during 'school hours' might in literal terms be against the rules of their contract, this makes little practical sense during a time when there effectively temporarily are no 'school hours'. Yes,no doubt they can still get in trouble for it, but calling it greedy or implying it's morally wrong is frankly ridiculous.
Incidentally, I am a teacher and earn from multiple jobs all the time. I work in 6 different schools and do private tutoring as well.