Extreme Stress Aat School

Short prepositions like "at" are not capitalised in titles. Words containing less than three or four letters are considered short, depending on the style guide you follow.

This week is the first week of 12th grade (In Germany the last year of school in Germany) for me.

(1) Unless the phrase inside the parentheses is a whole sentence, we do not capitalise the first word.
(2) I find it is clearer to put "in Germany" at the end.

Immediately I got to experience, how stressful this year would be.

The placement of the comma is awkward. It abruptly breaks the flow of the sentence.

Every single teacher's first sentences included, spoke of how short the semesters would be, and how fast we had to write our first test.

By definition, each teacher can only utter one "first sentence", so you should use the singuar form.

For context: because of the A level exams, both the semesters are shortened to only 4 months.

You can also use a comma instead of a colon.

Furthermore, in that time, there are many holidays and our school trip.

The addition of a comma would sound more natural.

Effectively, we have only 13 weeks to get 12x x 3 grades plus 12 class tests.

(1) I think placing spaces around the "x" makes it clearer that it's supposed to be a multiplication symbol.
(2) It might be helpful to the reader if you explain what "12" and "3" represent. (12 of what? 3 of what?)

On Friday, I'll already have to take tests in math and biology.

In addition, there are also a lot of projects that take a long time projects to do, like a herbarium, a gigantic computer science project and a recitation in gGerman class.

(1) It is not entirely clear what "long time projects" means.
(2) The names of languages like "German" are proper nouns and should be capitalised.

I really don't know, how I should balance all of that, while doing my driver's license, having to practice my instruments, teaching my instrument to others, do someing sports, and reading a book, and justill havinge some spare time.

(1) In "teaching my instrument to others", "instrument" is singular, so I changed the first instance of the word to be singular as well.
(2) Keep the tense consistent. You use "having", "teaching" and "reading", so you also use "doing" instead of "do".
(3) The general term is "sports". "Sport" would refer to a specific activity, such as in the phrase "badminton is a physical sport".
(4) Having spare time is a consequence of balancing all the other activities, so I think it is more appropriate to break it from the list of things you must balance.

This week is the first week of 12th grade (for me. In Germany, that's the last year of school) for me.

Immediately, I got to experience,d how stressful this year would be for me.

Every single teacher's first sentences included, how short the semesters would be, and how fastquickly/rapidly we had to write our first test.

For context: because of the A level exams, both the semesters are shortened to only 4four months due to the A level exams.

Furthermore in that timMeanwhile, there are many holidays andplus our school trip.

Effectivessentially, we have onlyonly have 13 weeks to get 12x×3 grades plus 12 class tests.

On Friday, I'll already have to take testexams in math and biology.

In addition, there are also a lot of long time-term projects to do, likefinish, including a herbarium, a gigantic computer science project, and a recitation in gmy German class.

I really don't know, how I should balance all of that, while domy schoolwork on top of getting my driver's license,. I also havinge to practice my instruments, teaching my instrument to others, do some sportexercise, reading a book, and just havinge some spare time.

In English, it's more natural to say to exercise as opposed to do some sport, which is a literal translation from German. If you want to say that you want to do sports, people are assuming that you're doing sports like soccer (We use that term instead of football in the United States.), swimming, running, and rock climbing.