<em>Here are some different ways to write this. </em>

English study

I am going to an English conversation school.
I have not spoken English until now.
So this English conversation school is good for me.
But I thought that I will give up this school on March 25.

(Or you can say, "But I thought that I will give it up on March 25")

Because my knowledge of the English grammar is insufficient,
I thought that I wanted go to an English grammar school after on March.

I am worrying about which one I should choose.

English study

I am going to make an English conversation at school.
I have not spoken English until now.
So this English conversation at school is good for me.
But I thought that I will give up this school on March 25.

Because my knowledge of the English grammar is insufficient.
I thought that I wanted go to an English grammar school after on March.

I am worrying about which one I should choose.

Nice job my friend. 
I really agree with Kamenko... not all English speakers know the English grammer. Also you can find some English native speakers who don't know the English grammer because he/she doesn't pay attention on it.

That's why I always say..nobody can speak any language perfectly...there's always something new we can learn.

If you need my help, feel free to contact me and reserve a class with me.