Which is Most Important Subject to Learn in School

English language. It is the most used in any situation when traveling for social and/ or business purposes. When you can speak English, you can get a job in the tourist industry anywhere in the world where there are tourists and they are mainly from the first world which is largely English speaking. English is even used to teach pinyin to Chinese kids when they first go to school. Imagine that! You have to use a foreign language to learn to speak your own language if you were a Chinese kid. Pinyin uses the English alphabet.

Calculus. 

  For most of the U.S. Universities, although they don't broadcast it, if you are weak in your math coming out of HS, then you won't fit in the initial queue for the Engineering Department. 

    Math in the U.S. is almost uniquely poorly taught in contrast to other subjects (after all, you WILL have to do homework), and it catches-up to most of the students when their High Schools cut them slack so they can continue being spoon-fed arithmetic and algebra. 

   This becomes a rude awakening during many students the first year at college when they realize they are stuck in the humanities, the bio-sciences or some other non-engineering major. You can't transfer to most Engineering Schools even within your own University.

Frankly, I think this depends on what is your aim. For example, if you want to become a doctor: Biology, English(or local language to communicate with patients), Mathematics is important. For becoming an Engineer: Physics, Mathematics, basics of Computer Science is important and of course, you still need to be able to communicate. Ad so on for other fields. But overall Mathematics and English are important for most fields. Out of these two English ranks higher(just a little) since it's a global language and you'd definitely want people to understand your ideas.

From my perspective the importance of the subjects depends on two factors:

1. Subjects you like.

2. Subjects that help to achieve the career you want to follow in the future.

3. Overal useful subjects: English, Spanish, Geography, Maths, among others.

Definitely only after you realize you want to do in life is that you will know which are the subjects you will find which ones you want to dedicate more time to. It is ideal when the subjects you like are aligned with your future aspirations, but sometimes they can differ

Communication is the most essential subject in education. Communication is a broad topic that encompasses many subjects, from reading, to writing, to speak, to the arts, to computer technology, to the sciences and many more. 

After all, language is at the heart of being human. Therefore, it touches and is touched by every academic discipline. Indeed, without good communication skills, even the most educated individuals are less valued in the workplace that are good communicators

Math is universal.

Unlike English or say, Korean, mathematics can be used in any part of the world. Math is also the foundation of science because math is necessary (Physics/Chemistry/etc).

Math makes you smarter.

Math trains you to think logically and critically. It's practically impossible to know every single mathematical formula, and because of that, you need to be able to think how a certain equation works and how it can be solved. Math also requires memorization (to some extent). This can train your brain to remember things because recalling information can be improved by... recalling information!

Math is used often.

Math is used daily. Mentally calculating discounts and taxes is probably something you do when you go shopping (if you don't, you should!). Without being able to do that, you wouldn't be able to save the most amount of money. Math is used even when involving physical labor. Let's say you have a group of buddies and are trying to move a certain number of bags to a location. With math, you can optimize the amount of bags each person carries even when involving more than one trips and an odd number of bags.

These are some of the reasons why I believe mathematics is the most important subject in school.