We have to do projects that last the whole school year. I'm in 7th grade. Do you have any ideas?

First off, I am not going to tell you what to do, but I will give you some advice.
First, I guess this depends on a few factors. How big must this be? If it's for a whole year, I imagine that an essay by it's self won't be satisfactory.
What are your interests? These might help you to decide too.
As a teacher, and using the limited information I have, I would be looking for the following.
Have you expanded your knowledge? (Don't just write or do something you already know. By the end of the assignment show how your knowledge of this area has grown.)
Is the amount you do equal to the time given? By this I mean your teacher has given you a large amount of time to do this task, therefore you should show that you have used your time wisely. Set yourself smaller tasks/goals that you can accomplish over the year and then bring them together to create a larger whole.
Another option could be to find something which enables you to make a difference. If you are allowed, find a cause you are interested in and work towards helping it.
Finally, I would suggest you think outside the box, do something which allows you to explore your passions in a new way.
I hope this helps.