...just a theory, and will l probably be shut down, but I've noticed that kids of teachers always seem to do rather well at school (both primary but especially at secondary). No offense but not all teachers are in the top 10/1% so it's clearly not all genetic. 'm sure there are exceptions but I suppose the teachers, even if not teaching their own kids, know what resources to look for and how answers should be structured and teaching in every day life which might not be something most parents are able to do effectively (and, yes, we all read to our kids, take them to museums and discuss stuff etc etc).
Now, to the 'stressful' part. I totally take onboard that the working conditions, reduced budgets, lack of salary increases, the constant testing, and changes in curriculum must make teachers' lives unbearably difficult a lot of the time.
Another aspect, and I've not seen this mentioned, which might play a part (for those teachers who are parents) is the increase in kids when going through 11+ (or similar) are nowadays tutored to a high degree (didn't used to be the case back in the day, you were either seen as 'grammar material' or you weren't).
So when teachers' kids go through the process as parents, it must put an extra burden on them. A couple of my kids had teachers whose children were in the 11+ year and, not being unkind, but those teachers were either super stressed or just not 'on it'. In a couple of cases, we had the same teachers over a few (different) years and the contrast in their application when their kids were not doing the 11+ was significant.
As a parent, I found it quite stressful and can imagine even more so as a teacher. It must add an extra stressor to their lives which is not really compatible with being a teacher (as I'm assuming they might also feel their kids can't fail the 11+ even if only subconsciously).