I think it depends on the circumstances. We were evicted from our home a while ago. By bailiffs. So we didn't have chance to grab more than the basics and leave. And even then I forgot stuff we needed.
When we contacted the council they could only put us in temporary accommodation 30 miles away and hope after a couple of days for a hostel closer to home. Luckily family came through for us in a pinch and I am forever grateful to them.
It is far from ideal to have a week off school unless ill and it sounds like the kids could probably do with the stability of school if. Itching else. A week off during gcse year might make a huge difference, especially if they are also doing extra revision sessions after school, like so many seem to do now.
I would move heaven and earth to get my kids go school. I'd ask friends and family to help out if needed because tbh I'd probably need the headspace to deal with whatever is going on without having to field questions from children as well.
I would, we did, approach the school and let them know what was happening. In part because we picked them up early that day to save extra travelling back and forth. They couldn't have been kinder. We didn't know where we would be staying at that point either.
If there really is no money to get the child(REN) to school, I don't know what else they could do but miss it. However, I would be concerned how much pressure - time and financially - a 40 mile round trip would put on someone in an already precarious situation. Once we were settled with relatives, we had an hours round trip twice a day to get the dc to school and back. I put no end of extra fuel in my car during those weeks of staying with our relative and was so grateful when we got a new home and were able to change schools. This isn't practical in gcse year though.