I've had both a positive and negative experience with it.
At my previous secondary school, I didn't get much help for it and kind of went under the radar so it was often never followed up upon if I had got help. If I remember correctly, I had one talk about it with a head of year in year 8 and it was never mentioned again and I wasn't offered much further help.
At my current sixth form, the help I've got is much better. I received some support sessions in school about it and my teachers were notified in a way I personally found appropriate and tailored to my needs (e.g not fully telling them all that was wrong but enough for them to be aware) and were assured I would still be working to the best ability I could manage at that time. The teacher I went to first about now asks me how I am and if I need to talk when she sees me around school which is nice.
At my current school it wasn't really discussed much with my teachers (although a few did ask about how I was doing and finding the work at that time) but the ones who did know dealt with it in a adequate way so I can't really fault them.
EDIT: looking at what was posted above me, I agree that there is very little teachers can do as they aren't mental health workers. That said it doesn't mean they shouldn't focus on it, it's just difficult when it's (what can be for some, at least) a complex issue.