Do I need to require a liability waiver?

I am taking some teenagers on a 4 night camping trip in about a month. It's not gonna be real rugged but there are still inherent risks to being outdoors and hours away from immediate help. And while I'm certified in first aid and I'm trained to handle the injuries that could happen, I'm wondering if I should require their parents to sign a liability waiver. Most of their parents already know me and have no problem with me taking their kids away for that long. I just worry that if something does happen, I'd be held liable. I've signed a lot of waivers for outdoor companies and such. Since this is a personal trip, I'm not under any umbrella. Not even with the church I know most of these boys from.