When you are at school, do you study biology or Biology? Are you in Sixth Form or sixth form? The rules for capital letters (the 'big' letters) are not based on logic. They are simply traditions. However, you should follow these traditions! If you do not, you risk appearing poorly educated. School, after all, is where education matters.
Here are some points to remember:
Do not capitalise the names of most school subjects. You study science, maths, history, and art. All these school subjects are in lowercase (non-capitals).
Capitalise subjects when they are the names of languages. This is an important exception to the rule in the previous bullet point. You study English, French, Spanish, Latin, Arabic, Mandarin, and so on. All these subjects are the names of languages, so all are capitalised.
Capitalise the names of courses. Students study many subjects in school. The subjects are often broken down into courses, which have names such as Introduction to Chemistry, Advanced Calculus, Poetry of William Wordsworth, and Environmental Politics. The names of courses should be capitalised. Do not capitalise every word, though. Unimportant words (of, the, a, and similar words) should be in lowercase, unless they appear as the first word in the name.
Do not capitalise the year in school. You may be in year 9, but not Year 9. Why? There is no good reason for this tradition. This practice has existed for a long time. Perhaps, long ago, a teacher wished to show the students that they were not important enough to merit a capital letter!
Capitalise the titles of school assignments. Not every piece of work you write for school has a title. Essays, reports, presentations, and research papers usually do have titles. Capitalise the first word and all important words of the title when you are writing for an English, history, art, or language class.
If you are writing for a science class, capitalise only the first word of the title and any proper names. Scientists like to keep things simple!
Remember these rules and you will have a better chance of achieving high marks.