Well, like many others have said... stop studying in groups of people who don't know anything more than you do.
I gave up on study groups when I was in UG. When I went through law school, I studied on my own. You will probably have several projects in your 2L and 3L years when you have to work in a group or with a partner. But for now, study on your own. (without distractions)
Leaving exams early:
I left the multiple choice exams early when I finished before time was called because I was confident in my answers and knew that I would not change my answers. My multiple choice scores were always either #1 or #2 in the class. However, I've seen enough spelling and grammar issues in model answers (law school essays and Bar essays) that I would never leave an essay exam early! If you truly cannot think of any more analysis to add to your essay then go through and fix the grammar and spelling issues before the proctor calls time.
Keep in mind, the bulk of your points are always going to be in the analysis! Your favorite word should become "because." (i.e. this was not a burglary BECAUSE Defendant did not have the specific intent to commit a felony once inside) (i.e. Here, there was no valid contract BECAUSE there was no acceptance of the offer). Most, if not all, of your classmates will have memorized the black letter law... But, the ones who do exceptionally well are the ones who can apply the black letter law to a new set of facts. In short, the analysis should be as full, complete and thorough as humanly possible.
About your legal writing:
Do whatever your legal writing professor wants from you to get a good grade in that class. I landed my first law firm job because of my writing samples. You will probably use your legal writing assignments for your writing samples because you don't really have anything else to provide an employer this December (assuming you want a job).
On that note, after your assignments have been graded your school probably has writing tutors who will help you fine tune the piece to use as a writing sample. No matter what grade you get, use the tutors!
Prior legal knowledge:
I failed a required class in law school because I got a C in the class. That means I had to repeat that class. Here's the crazy part of it. The class was criminal law. Criminal law was the only area of law that I had experience in when I began law school. I could rattle off the elements of burglary etc., from the top of my head. I literally taught my Crim Law Professor something about criminal law that she was unaware of during one class. So, how did I fail the class? Because I did not follow my professor's instructions for the Final Exam!!!
In the first class the professor told us to memorize the cases and use them on the exam. I remember thinking "I know criminal law, I don't need to memorize these cases."
Let my hardship be a lesson to you... Do everything your professors tell you to do, exactly how they say to do it.
When you begin studying for the Bar Exam, take a Bar Prep course and follow their format for essays exactly...
You'll be fine.