First of all, you should rest assured that compared to any other kind of writing, school essays are a breeze. You don't need to be particularly bright or skillfull to pull them off. You just need to be able to read the question - and most of the work is already done.
You can get away with around 500 words too - which is pretty short. You can write up to around 2000 words if you want to look like some kind of a swot. But padding out an essay with extra words really doesn't cut it - or impress the teacher - so my advice is to keep it short.
A good essay is broken down into 5 parts:
2. Terms of reference
Basically five paragraphs of around 100 words each, which equates to about 2 or 3 sentences in each paragraph, tops.
Now let's look at how you fill that word count.
1. In the Intro you basically need to restate the question - to show you understand it and then make some sort of reference to the fact that you're going to answer it in the essay.
So, say you have a question about what makes Peru an important country, you would say something like: Peru is a fascinating country and this is an interesting question because Peru is important, as I shall outline below...etc
2. Next, within the terms of reference, you need to define the context in which the question will be answered. Here's where you tell the teacher what you already know - even if you feel like you're stating the obvious.
In the above example you might explain that Peru is a place in South America, that a country is a self contained economy and that, compared to the whole wide world, it's small potatoes...etc
3. In the third paragraph you need to deal with the question. It's best to have at least two facts up your sleeve to drop into the essay at this point.
But don't panic.
Look again at the question. Why are you being asked it? What's the most obvious answer? In the case of Peru - you might guess it's important because of its culture (a good answer to any question by the way), therefore tourism is going to be a factor - and maybe its because the people invented chocolate, sacrificial pyramids and cocaine - all valid things to bring into the essay.
4. In the fourth paragraph you need only discuss the relevance of the things you mentioned in the third paragraph.
For instance you might mention how chocolate is popular because it's tasty, how cocaine is a bad thing because it kills people, how pulling out people's hearts on sacrificial altars probably seemed like a good idea at the time etc etc.
Easy, but even easier is the:
5. Conclusion. Here you restate the question and tell the teacher you've answered it and proved your answer by the repeating the statements you've already made.
You might say, in conclusion, Peru is an important country because of its chocolate and cocaine production and because tourists like to visit the pyramids and spend their money there.
Voila - an essay likely to receive a gold star.
Here's a tip. Teachers know this essay structure like the back their hand. Therefore it's unlikely they'll do anything but skim your essay looking for the two or three points you're making. So there's really no point to filling your essay with lots of interesting and insightful facts - it will only confuse them and cause them to work harder, which they don't like doing.
Keep your essay short and sweet and if in doubt, waffle. Teachers, at the end of the day, prefer to see that you've tried, no matter what your answer.
I hope this helps.