This has happened to me three times in my career, and I don't think it's anything you can plan for. For those first few days, you need to expect to be fairly fluid and take things as they come. Let the students know that different people deal with grief in different ways, and that they must be kind to each other and not bicker or criticise. It's also worth being on the look out for "competitive" responses - I know that sound heartless, but it need to be said (not to the kids, I don't mean that) The first time this happened in a school I teach in was several years ago, when a Yr 9 boy was killed in an accident on bonfire night. He was a bit of a likely lad, and so his friends were the "naughty" boys. It was heartbreaking watching them try to deal with their grief, but also seeing how it was exacerbated by the (mostly) girls who suddenly decided that the poor boy who had died was their best friend.
The third time, which is far more recent, was a girl who committed suicide. She was Yr 11, and there was a lot of anger from those who were genuinely her friends feeling that people who had not been kind to her were jumping on the grieving bandwagon. The anger went on for months - to the point that their end of Year 11 assembly was disrupted when the Head tried to pay tribute to her, and those who felt she had not been dealt with fairly at school when she was alive made a bit (and unnecessary and unfortunate) scene.
The second time was a wonderful boy in Yr 13 who died of sudden adult death syndrome. That was terribly sad, but the grieving was more "mature" (if I can say that) because our 6th form is small and very close knit, so the students were very supportive of each other.
Just try to be there for the ones who need you. I'm sure the school will have a plan.
Such a sad event, but I guess it's a rare teacher who doesn't have to deal with this during a career.