A group of high school seniors head out to a lake house for a weekend to celebrate graduation. Things get out of hand after several of them begin using an illegal substance that opens the door for demonic possession and ultimately threatens their very existence.

I like what you have but you need to say more about the substance, maybe how they came to get it? Why it leads to demonic possession. Good one the less, I get an evil dead feel kind of.

Agreed with all the above.

In addition these characters are the ones who created the problem in the first place - they need to clean up their own mess. Best if the inciting incident is forced upon them, this means that they must proactively take action to save others and be heroes instead of silly teenagers that screwed up.

Seeing as there is such a long lineage of teens in the woods horror movies from Halloween to The Blair Witch Project, how is this going to provide a fresh take on the old concept?
I'm not saying it can't be done - the genre is a staple. Consider Cabin in the Woods it is a great example of a spin on the genre yet still observed its conventions.

The logline presents the characters as hapless, passive victims of circumstances "things get out of hand..." , "opens the door to demonic possession...", "threatens their very existence". ?A logline should describe how a protagonist proactively struggles against the adversity that befalls him.