I think this thread got deleted as I was posting a response... but the following is worth sharing which I was typing as the delete happened...
I don't know how much contact some posters have had with people who've had mental health issues in the past. Suffice it to say, most people are probably aware that around 1 in 4 of the adult population will have some mental health issues in their lives.
Bipolar disorder is relatively uncommon (somewhere between 3 and 15 people in 1,000 will have this diagnosis in the UK in their lifetimes). However, it's a disease which is very well understood and usually very well managed by mental health support professionals. The majority of people with a diagnosis will find a medication regimen which works for them, and they are then able to lead happy, healthy lives. The vast majority know what their own triggers and risks are and to safeguard their own mental health and stability don't put themselves at unnecessary risks.
For a teacher to be outed with this diagnosis suggests gossiping which is both ugly and unproductive. He/she will have to disclose their diagnosis to the HT and will almost certainly be on a closer support level than colleagues without a diagnosis would be. Therefore any cycling or lapses into unhealthy behaviour are very likely to be picked up by either the teacher themselves or their manager long before it has any kind of impact on DDs or teaching practice.
It would probably be a good idea, if you find out a teacher has this diagnosis and are scared about the potential impact on your children to find out more about bipolar disorder and how it has very different impacts on different people, most of whom manage it really successfully. Teachers are risk assessed to within an inch of their lives; I think you really don't have much to be concerned about here.
My sources are working in an organisation which supports people in mental health crises and two close relatives with this diagnosis.
Posting about issues like this in AIBU (where I think the average level of understanding of MH/SEN issues is a lot higher than the general public) isn't likely to receive a particularly sympathetic response, but it will be a better informed one. But most posters don't have the time to post all this to those who are worried without reading more about it, here's some
Just trying to avoid anyone becoming a vigilante/witch hunt leader...