"My understanding through my SIL (teacher) is that teachers are not assessed enough and poor to satisfactory teaching is extremely common."
What rubbish.
All teachers are assessed numerous times throughout every term in various ways, as part of their performance management. We have several lesson observations throughout the year, one each term minimum. These will be conducted by the Head, senior management, and subject coordinators. Not to mention Ofsted though thankfully that doesn't happen too often! There are also learning walks and times when senior management simply drop-in on lessons (almost a weekly occurrence in my school).
Most schools have weekly staff meetings that underline key areas for teaching and identify any areas that the school needs to develop.
Then there is the constant assessment of children and associated paperwork. We have to complete an evaluation form for every child at the end of every core lesson, along with updating the child's personal targets whenever an objective is achieved. We level up the children at the end of each half term, and yes, we are held accountable if any of them are slipping behind, even if their performance is down to factors unrelated to the teacher (poverty, trauma etc). These children are identified through pupil progress meetings and are then offered extra support where necessary. If a child is not identified early enough and does not hit their targets by the end of the year then the teacher will have to explain why.
As a teacher I am held accountable for almost every single hour of my working life in the form of hourly objectives, success criteria, tick lists, marking, and evaluation forms. I very often don't have time to plan "motivating" lessons, though I would desperately love to. It's why we do the job, after all.
If your school is allowing mediocre teaching then it speaks volumes about the management. Of course there are bad teachers out there, there are bad apples in every profession, but in general I have found that if there are a "number" of bad teachers in a particular school then it's down to management.
Either the management is so weak and silent they are not providing enough support for their staff, or they are coming on too strong and are stressing their staff out to the point where they don't have the energy to be "motivating" anymore.
To say that teachers aren't assessed enough is rubbish.