A group of high school kids discover how to time travel, but fail to recognize the potential consequences.

Way too vague for me. Sounds like Bill and Ted's Excellent Adventure.

2/8/13 5.30am test.

This is "Situation over Story."

If we did the same thing for 'Back to the Future' we'd get almost the exact same logline.

There's an interesting idea here but it needs a main character and it needs to be more specific.

That's a bit vague.

I suggest that, though there is a group of kids, you focus on their leader for the logline, and just mention he's not alone. And then establish what his goal is. So, he and his friends, (or she and her friends,) have discovered time travel, but what do they intend to use it for? Who or what is trying to stop them? ... Are the consequences the antagonistic force, if so you could still be a bit more specific. And what are the stakes? What happens if they fail to achieve their goal?