I am looking for the name of a movie probably made no earlier than 2000. Possibly British. A group of kids (teens?) go on a trip with an older woman (teacher?) into a remote marshy area. They encounter a group of ruffians, and after they complain about noise, things escalate. Its not "Eden Lake" or "The Reeds", but is similar to both in tone. The one scene I remember is when the bad guys kidnap the teacher and tie her up and thrown her alive into the lake with a weight on her, in front of her horrified students. From there, it becomes a bloodbath as you would expect. Thanks!