Oh wow. Finals in eighth grade? My suggestions are that you should find something to motivate you at times when you don't feel like studying. Which, in your case, is to get good grades on your final. Does your school offer after-school tutoring? Maybe, during this week, you could stay after and get some help. You could also ask your teachers. They may seem strict, but most teachers want students to do well. Find spots where you have trouble and ask for a bit of guidance.
As for your parents, find a time to talk it over, like at dinner. If you don't work best under pressure, then they should know. Explain calmly that you don't work best under pressure (If you say that you do, you're probably lying to yourself. Almost no one works well under pressure.) and that you could really use the extra study help.
Another thing, don't cram all of your studying in at once. Divide up your study time (Ex. Civics one day, Physics the next day, etc,.). I suggest planning it out chronologically. And I like to listen to music while studying or doing homework, but listen to instrumental music. I highly recommend classical music (Mozart or soft music can help you study better and keep you focused.) And try not to have sugary snacks while studying. Sugar and studying usually don't mix.
Hope this helps and good luck! :)