My daughter was a student at the school the first year I taught there, and she had her friends over for sleepovers and stuff, so it's been open to other students from the start.
I did talk to the mom in their driveway when I picked her up yesterday, and today I emailed the principal and just let him know how she's doing in her coursework, and mentioned that she was here, so I've covered my bases there. I'm sure it's fine, but it's still better to make sure everything's out in the open. (And mostly I wanted to make sure the principal knows she's putting in a ton of effort, because sometimes all they see is a student being late to class every day, they don't know the work going on behind the scenes.)
But also it's part of the culture of the school, we're a little more relaxed in some ways because we're a small school. I know the principal has driven kids home before when they needed rides, I drive kids daily, other teachers have as well. In a different school, it probably would put my job at risk or at least get me some reprimands.