If you plan on studying in the US, you better learn how to write an essay. Essays and other writing assignments often comprise up to 2/3rds your grade in a given course.

The most difficult type of essay is the exam essay. There, you must write extempore without the use of notes and within a time limit.

Here's an article from lifehack.org that professes to do just that, teach you how to write an exam essay.