Your performance in academic exams plays a very large part in your ability to get into a good college, and staying in it. It can be extremely competitive, especially for larger schools or more difficult courses. You will have to study hard and develop the skills and concentration to use that time efficiently.
Whatever the work involved, it’s worth it. In some cases, your exam can be the biggest basis for your grades. Not all teachers will really give a lot of weight to class recitations or projects. Quite a few will schedule a midterm and a final exam, and in those two tests lie your future.
There is a common misconception that studying for an exam refers to the two or three days before the test when you are feverishly memorizing your notes and books. Those days are just a review of the subject matter. Ideally you have already mastered it. You have attended all the classes and done all the reading homework already. You have made your own notes and at least comprehend the basic subject matter, and can talk about it intelligently.
But if you have skipped classes, never taken down your own notes, or still feel as ignorant about the topic as you did when you first enrolled for it, you’re in trouble. No amount of cramming (and using other people’s notes too!) can make up for the hours of “study time” you missed throughout the semester. The curriculum was meant to be mastered over several months to work, not several days.
And that point becomes very very clear, in fact, when preparing for college entrance exams. When you prepare for an entrance exam, you are being tested for the skills you were supposed to learn in the high school curriculum. Technically, the most you can do is to go through a “refresher course”. Some high schools do, in fact, offer a few weeks of review. But the pace of these classes are very fast. It should give you an idea of the topics to focus on during your personal study time, and also to help you practice handling the pressure of the exam.