I am looking for a horror movie with a similar plot

I haven't seen it since I was a kid, so forgive me if I get some of the details wrong. My guess is that the movie is from the late 70's or 80's.

I believe it was a school bus with students that was traveling through a forest when it breaks down. After that, someone starts attacking the students. Must be in the middle of nowhere because there didn't seem to be any hope of help coming soon.

They decide to fight back, setting traps and probably making weapons. I think they might have suspected the bus driver who was a bit odd but it wasn't him. I think it turns out that it was the mother of bus driver or someone else on the trip. I forget why she came to be out in the wild but she had been there for years. When the kids finally win, whoever was her kid mourns her and maybe even the kids feel a bit bad after seeing this.

I think the killer had long hair and was covered in moss or plants, looking like a camouflaged wild person. I really enjoyed it as a kid because they fought back and didn't act like idiots, a theme in almost all horror movies that I like. If you know the movie, try not to give to many details because I'd like to see it again without knowing all the stuff that I forgot.