Some people thinkWho is responsible for a child's social education?

School is a definitely an essential part of everyone's life.

Definitely is more often used as an adverb than an adjective. Therefore it should sit next to "is" and not between "an" and "essential".

There are so many opportunities for self-grouwth, personal development and improving your social skills.

When you talk about people in general, it's more common to use 2nd person (you/yours) than 1st person (we/ours)

If you are a lucky person, you will have a positive school experience.

"If you are lucky" is much more idiomatic than "if you are a lucky person" – but the latter is not incorrect.

Personally speaking, my school period isdays were the best oneperiod in my life.

"school days" is an idiomatic phrase

I had incredibly professional teachers, who wasere always ready to support me and give a good advice.

Besides, I was interested in a variety of sports and extracurricular activities so I was always was like a leader in friends' companiea leader among my friends.

"always was" is not incorrect, but it's more common to say "was always". When you want to use "company" to mean being with your friends, it's always singular. This is a less common term however.

If I hadn't haved all of these things in my childhood, I would have been unhappier.

This is a complex verb structure, well done! You were almost perfectly correct.

I believe that every parent should be prepared for a real life and realise that bringing- up a child and the process of adaptation the child adapting theo society is absolutely his/their responsibility.

I wasn't exactly sure what you meant in the last part, so I guessed. It's extremely common even in formal English to use "their" instead of "his/her" when you're referring to someone whose gender is uncertain or could be any gender. You will likely only see "his/her" in super-formal documents like contracts and government policies.

There are so many opportunities for self-grouwth, personal development and improving our social skills.

Just a little spelling mistake

Personally speaking, school period iwas the best one in my life.

Since you are speaking about the past, you say was

I had incredibly professional teachers, who wasere always ready to support and give a good advice.

As it is plural you say 'were'

Besides, I was interested in a variety of sports and extracurricular activities so I was always was like a leader in friends' companies.

make it plural

If I hadidn't have all of these things in my childhood, I would have been unhappier.

I believe that every parent should be prepared for a real life and realise that bringing-up and the process of adaptation of the child in the society is absolutely his/her responsibility.