I want to build on Kleo Ng's correction. 

For your paragraph on your high school academics, I would phrase it like this:

My three years in high school has enabled me to acquire extensive knowledge in history, geography, French literature, philosophy, and modern languages. In history, I learned about key events in French history, different French governments, different French presidents, the Second World War, and the Cold War. In geography, I learned about ... (fill this in) In French literature, I learned to appreciate the famous works of various French authors, including Pierre Corneille, Jean de la Fontaine, Molière, Blaise Pascal, Jean-Jacques Rousseau and François-René de Chateaubriand. In philosophy, we discussed the works of Plato, Aristote, Cicéron, Descartes, Freud, and Spinoza. Finally, in modern languages, I acquired basic/intermediate/advanced English and basic/intermediate/advanced Spanish.

You asked how to avoid parentheses. Kleo Ng removed your parentheses but created a super long sentence. It was very hard for me to read that sentence. To avoid both parentheses and long sentences, I propose introductory sentences. In my version of your paragraph, the first sentence gives an overview, and the other sentences fill in the picture.

You might find my version of your paragraph too long. In that case, I suggest the following version:

My three years in high school has enabled me to acquire extensive knowledge in French history, recent world history, geography, French literature, philosophy, English, and Spanish.

Do you think the school really needs to know what exact topics you studied in history, French literature, and philosophy? Up to you.

Since you've already been offered an excellent correction, I would just make some stylistic suggestions based on what I've written for university and other formal applications. Also, I removed most of your parentheses using semi-colons and colons to list and organise your clauses. Hope this helps! (blue: suggested corrections, grey: optional additions, turquoise: some ramblings) :) 

Letter for a university (part one)

Dear Sir or Madam,

I am writing to apply for a PGDE in modern languages (French) commencing in the 2014-2015 academic year. I have had much experience in the domain of education and this field is one in which I have much interest. 

I am a native speaker of French with extensive language-teaching experience. Born and brought up in France, I had a predominantly French education, and this is one reason why I have decided to venture out of France to widen my horizons; an experience that would greatly aid my future career path (a suggestion to perhaps "market" yourself a little more as the right candidate!).

I passed my baccalaureate with merit in 2009, the equivalent of A levels in the United Kingdom, in French literature, as well as English and Spanish as foreign modern languages. My three years at high school has enabled me to acquire extensive knowledge in: the history of France, which also increased my understanding of key events in French history like: the different French governments and the French presidents, and the most important international conflicts such as the Second World War or the Cold War; in geography (where we had the opportunity to learn about different cultures around the world?); in French literature where we learnt to appreciate the famosu works of various French authors like: Pierre Corneille, Jean de la Fontaine, Molière, Blaise Pascal, Jean-Jacques Rousseau and François-René de Chateaubriand; in philosophy where we discussed the works of Plato, Aristote, Cicéron, Descartes, Freud(did he have philosophical works? I thought he was a psychologist! Just curious :) and Spinoza; and finally, in modern languages where we honed our skills in (I wouldn't say oral, do you mean comprehension or the spoken language?), listening and writing.

My studies were mainly focused on literature and my strong interest for modern languages encouraged me to apply for a university degree in the field of literature and modern languages. My course (LLCE, langues, littérature, civilisation étrangère) placed great emphasis on English philology, where I studied the most important English-American literary works including: Poe, Defoe, Austen, Shakespeare, Auster, and the civilisation of the main English-speaking countries (the first colonies in America, the Civil war, the Commonwealth, the Scottish War of independence, the conflict between protestants and catholics in Ireland).

The extensive study of the language (grammar, linguistics, phonetics) teaches students how to become English teachers, which had always been a part of my plan.
Since passing my degree in 2012, I have wanted to put the theory I learnt into pratice by living one year in an English speaking country. I have always loved looking after children and I used to babysit frequently during my time as a student in France, so working as an au pair seemed to be the perfect experience to live for me. (I took up the challenge of working as an au pair as it seemed like the best way to practice the language. The babysitting experience, alongside formal lessons, offered me many opportunities to converse in English, ** did this experience affect your motivation to teach? if so you could say: (insert event, for eg. teaching the kids french/english?.. and this experience has ultimately firmed my belief that teaching is a profession I should seek.)

do let me know if there's anything else you might need me to clarify! cheers :)

Letter for a university (part one)

Dear Sir or Madam,

With many experiences in education and being very interested in this field, I am pleased to present my application for a PGDE in modern languages (French) for the school year 2014-2015.
I am a native speaker of French with extensive language-teaching experience. I was born in France and studied mainly in France. Indeed, I passed my baccalaureate with merit (it could be said to be roughly the equivalent of A levels in the United Kingdom) in 2009 in French literature (including English and Spanish as foreign modern languages). Therefore, my three years at high school enabled me to extend particularly my knowledge in the history of France in particular: the different French governments and the French presidents, the most important international conflicts such as the Seconde World War or the Cold War.  I also expanded my knowledge of geography, in French literature (Pierre Corneille, Jean de la Fontaine, Molière, Blaise Pascal, Jean-Jacques Rousseau, François-René de Chateaubriand), in philosophy (Platon, Aristote, Cicéron, Descartes, Freud, Spinoza), and in modern languages (oral, listening and writing).
My studies focused on literature and my strong interest in modern languages encouraged me to embark on a university degree in the field of literature and modern languages. My courses (LLCE, langues, littérature, civilisation étrangère) emphasised on English philology. I studied the most important literary English-American works (Poe, Defoe, Austen, Shakespeare, Auster) and the civilisation of the main English-speaking countries (the first colonies in America, the Civil war, the Commonwealth, the Scottish War of independence, the conflict between protestants and catholics in Ireland).
The extensive study of the language (grammar, linguistics, phonetics) teaches students how to become English teachers, which was always my plan.
After passing my degree in 2012, I wanted to put theory into pratice by living one year in an English-speaking country. I always loved looking after children (I used to babysit quite often during my time as a student in France), so working as an au pair seemed to be the perfect experience to live for me.

[Good job.]