My DS has used a weighted vest with his OT. It's something I've been debating whether or not to use at home or school at all.
On the plus side, it gives him about 20 minutes or so where he can sit (relatively) still and focus on a task. But beyond 20-30 minutes, his body gets used to the weight and he doesn't notice it anymore and he starts getting squirmy, fidgety, clumsy again. And as soon as the vest come off, he does have a harder time with his focus and spatial awareness.
At this point, I don't think it would be helpful at school because the negatives (especially the backsliding afterwards) would outweigh the benefit. At home, right now I am able to provide him enough other accommodations and tools that it isn't necessary. If it got to the point where he needed it I would probably put one together but for right now, I don't see a clearcut *need* for it.