Well, I teach kindergarten, and even if I keep my hands off the children, they don't always keep their hands off me! I have had children---often---just come up and fling their arms around me for a hug. I have had them crawl onto my lap if I am sitting close enough. I often have to hold their hand if we are going down the stairs because some of them aren't too steady
That said, with my older students, I definitely do not get them doing those things, and I definitely do not initiate. As far as hard-core rules, the only thing we have been specifically told is that if a child has a bathroom accident, we don't have to change them. Usually, we do---they wear uniforms at our school and there is a cupboard full of spares, and none of us really want to leave a kid sitting in that if we don't have to. But the school 'policy' is to call the parents and ask them to come and change their kid. If we do find ourselves in a situation where we assisting with such a thing, we've been told to leave the bathroom door ajar so we are not alone with a child, and I know our two male teachers are especially cognizant of this.
I had one situation where it was after school and I was running after-care. We had a child that year who was chronic with this sort of thing, and it came up during my day to do this. Our admin was thankfully still there and she stayed with me while I helped him change, but if she had not been there---since it was after hours and other teachers were not around---then yeah, in that case, I am afraid I would have let him sit in it. I am just not comfortable with that sort of situation to be on my own with it. And everybody---even the child's mother---would have understood that and not been upset.