If you did your best to write a complete, clear, and insightful essay within the time allotted, you should have nothing to worry about.
Your best supporting idea – the one that most strongly makes your case and, simultaneously, about which you have the most knowledge – should go first. Even the best-written essays can fail because of ineffectively placed arguments.
On the topic of planning, it`s important to sketch out an idea of how long you want to spend on each section of your essay. If you know the number of paragraphs you`ll need to write ahead of time, you can do this before the exam even starts!
However, it`s important to keep in mind that your professor understands the circumstances under which the essay was written. They`re fully aware of the time pressure you were dealing with, and they will judge your work far differently than they would judge a typical essay with a deadline set weeks after the assignment date.
It`s important to give context that will help your reader understand your argument. This might involve providing background information, giving an overview of important academic work or debates on the topic, and explaining difficult terms. Don`t provide too much detail in the introduction—you can elaborate in the body of your essay.
Start planning if you can. Although the situation described above sometimes occurs, it`s also very common for professors to give their students a fairly detailed idea of what an essay question will involve in advance of the test day.