Does this sound natural?

After this holiday I'm going to the third grade of high school. I attend something like biology class, I study biology, chemistry and math on an advanced level. Those are some of my favourite subjects. The other one I like is English, and my plan is to become fluent, but I have to study by myself. I used to attend private classes for 2 years but I learned only the basics and I wasn't able to communicate with others. Now I have a wonderful teacher, but according to the core curriculum, she teaches us English on an intermediate level. Since I got involved in learning, I'm getting better and better in English, nevertheless, there's always something to learn.

It’s all grammatically correct, but the a lot of these sentences end when they can simply be joined. Here’s another interpretation of your paragraph:

When this holiday ends I’ll be going to the third grade of high school. I attend something similar to biology class - I study biology, chemistry and maths, on an advanced level - which are some of my favorite subjects. I also like English and my plan is to be one fluent, however o have to study by myself. I attended private classes for 2 years but learnt only the basics and was unable to communicate with others. I now have a wonderful teacher, who teaches English on an intermediate level according to the core curriculum. Since I got into learning I’m only getting better in English, but I can always improve.