I've had one in the past. It was fine. They asked what they could do to support me on my return. Sometimes you can get a phased (part time or reduced duties) return - but beware, when you're signed off the sick you are declaring you are fit to do every part of your job.
Occupational Health in teaching, I was told by my headteacher in one or our less pleasant meetings, is the school's way of gaining evidence in case they need to dismiss you for incompetence. Anxiety is a disability which is sufficient to prevent a teacher from being able to do the job properly (you can tell, I've been all through this).
I fought them for years. Years. But the last five years they put real effort into making it impossible for me to do my job, so I've resigned. No job from 1 September, and until then, off sick.
You need to get the union onside now, before you return. Maybe everything will go well. But if your school is like mine (or is mine, even, as we are anonymous here) you'll need an advocate in due course. I didn't have one. Other events in my life went tits up and I stopped fighting.
Good luck. I got seventeen years pay for seventeen years work, and it was better than I'd have earned in a shop or office. They beat me in the end, but I finished paying for my daughter's education, paid for her wedding, and hopefully will very soon have paid off my mortgage. I'm only bitter if pressed. :)