I know of someone who was off sick for mental health reasons. They felt that if they went out, and people saw them, that people would think they were well enough to be back at work.
There were times when they had no choice but to go out, and the people they bumped into didn't judge as they thought. They all saw going out of the house, and doing every day things, such as shopping, gardening, a meal etc as a positive step to feeling better.
I was told by a GP the benefit of walks and exercise on mental health. I also know of health boards that have put booklets together suggesting places to walk and things to do, specifically with mental health in mind.
I persinally have taken leave/been on holiday before, because I felt I needed it for my mental health. I knew I needed time to do things for me, and my mental health benefitted as a result. Neither time did I feel the need to be off sick, but if I hadn't put myself first and taken that time then that could have been the result. If others had felt how I did, I may have suggested they take time off sick.
If I knew of someone off ill for mental health reasons , and they had gone on holiday I would probably see it as them getting rest/relaxation, they needed. A festival, I may/may not feel the same way, but it is still a way people telex and enjoy themselves, which has positive benefits.
You have said you work in a school, but we don't know the job of the person in question. Being well enough to take part in normal leisure activities, is totally different to being well enough to be responsible for 20 + children at one time.
There will always be people that play the system, but i wouldn't assume that to be the case. Maybe I wouldn't be posting on social media what i was doing, but if they are not doing anything wrong, then they probably don't see wht they shouldn't.