Holidays/festivals whilst on the sick

They would have been very wrong then.
You are signed off sick from work, not signed off from life.

Your manager should stick to their own business.

If you are single you would have to go to the supermarket and it's none of your manager's business.

I fully disagree, I went on holiday after my mum died and I was "off sick" and I cried most of my time there.
Have you ever actually been off for mental health?
You're comparing mental health with a stomach bug and they would be handled very differently. If someone is off with mental health do you expect people to just sit at home in a dark room and not try do these to help?

the one and only time I was signed off sick from work for two weeks I was told by my manager that I wasn’t allowed to leave the house except for medical appointments and that if I did it would be classed as gross misconduct

YABU
They will be doing it anyway. The only difference is that you see it on their social media.
Are they meant to be posting pictures of themselves crying instead?
Getting out and doing things is good for mental health, and the fact they are doing those things is a sign things are looking brighter for them, and they could be back to work soon.
You do not go from being ill, to being ready for work overnight.
When you have MH issues, the worst thing you can do isolate at home.

Working in a school and attending a festival are clearly going to have very different demands on your mental health. You can do whatever helps your health whilst off sick, if that’s a holiday or a festival, that’s fine. If it’s working in a different school whilst off sick then obviously not fine.

I think if you are off because you claim you can’t go in rather than you just physically cannot do the job then yeah it takes the piss to be on holiday and festivals and such.

If you are home because you cannot do your actual job because of a broken leg or hand then fair enough.

They were daft to post on social media given the judgement they're getting. The activities they undertake while off sick with mental health issues are none of your business but given your reaction they'd be wise not to give their colleagues ammunition.

That seems wrong to me.
Are people with broken legs meant to stay at home until they are 100% ready to return to work?

I think it largely depends on the reason your off work. If your off work with a bad back / physical injury and stating you can’t possibly get out of bed then yeah your taking the piss a bit. Mental health is different though, people have to do things that make them happy in order to feel better. You can’t improve your mental health by isolating at home alone.

I think it's out of order. A friend got caught out at a community event in a local park while she was off sick with a broken leg. She couldn't walk but did manage to hobble there on crutches. She was seen by other staff and long story short, they got rid of her

Yanbu about festivals but going for walks and meeting a few friends for coffee really helped me when I had depression.