After I took the middle school entrance examination in our hometown, I was enrolled in the township middle school.

"I was enrolled" sounds like it wasn't your choice to do it; "I enrolled" feels more idiomatic to me, but this might just be a cultural difference. Both versions are grammatically correct—it just depends on what you want to emphasize. Including the word "was" gives the impression that the choice was made for you by your parents or by someone grading the exams/working for the schools.

That year, three classes of grade 1, about 180 new students, were enrolled.

You might want to specify "about 180 new students in total" or "totaling about 180 new students", if that's what you mean; otherwise, it's a little bit ambiguous as to whether it was 180 total students or 180 per class (for a total of 540). If it was 180 per class, you could say "with about 180 new students in each".

The three classes were marked by aA, bB, cC and I was in class aA.

"marked as", "labeled", or "designated" might sound more natural

We had eleven courses and they, which were Chinese, Mathematics, English, Politics, History, Geography, Physics, Chemistry, Biology, Music, and sSports.

If "sports" was one single class, then it should probably be capitalized like the others; if your 11th class was always some kind of sport but not always the same one (for example, if you had basketball one year and soccer the next year), you might want to write "and a sport" or "and different sports" instead of just "and sports", to make it clear that it's not just a capitalization mistake.

The school had dormitories, so we stayed inat the school during our studying dayies.

"stay in school" = idiomatic expression meaning "keep regularly going to school (on a longer timescale, like months or years)"/"finish your degree"; "stay at the school" = reside there, live there while studying; "stay in the school" = stay indoors inside the school building the entire time, not leaving at all. "Studying days" is not really wrong, but sounds old-fashioned.

Every day, apart from the formal six classes (four in the morning and two in the afternoon), we also had the morning independent study andin the night independent studymornings and at night.

Just a minor/optional correction for flow. If it was a time period where it was mandatory to attend and study, then "we also had independent study" or "we also had independent study periods" sounds right, but if it was free time that you chose to spend studying on your own, then "we also studied independently" is more natural.

Studying in middle school was mya highlight tifor me because I got first place in exams several times.

When I graduated from the middle school and took the high school entrance examination, I got first place again.

It's fine as-is, but "graduated from middle school" sounds more idiomatic.

But I didn't choose to go to a key provincial high school,; instead, I went to a specialized secondary school.

You could also split this into two separate sentences if you wanted. (By the way, this is a mistake that even native speakers of English make a lot when they're writing!) More info: https://en.wikipedia.org/wiki/Comma_splice

The reason was that going to a specialized secondary school meant I already got the chance to work in the local government.

"Already" is a little bit vague here—I'm guessing you mean that you started working in local government immediately after finishing secondary school, but the phrasing almost makes it sound like you were working in the government while you were still studying. If my guess is right, I recommend phrasing this as "…meant I got the chance to work in the local government as soon as I graduated", or maybe "…meant I got the chance to work in the local government while other students were still studying", if you want to emphasize that you were already working while they were attending high school and university.

That choice left me with a biglot of regret in my life forabout not having the opportunity to study in high school and university.

In the correction, I tried to stay as close to your original phrasing as I could, but if I were writing this sentence from scratch, I'd probably write something like this: "Although I benefited from that choice, I regret not having the opportunity to attend high school and university." The "although…" part isn't necessary, but I think the text flows better if you do, because it makes it obvious how this sentence is related to the previous one.

"Study in…" sounds a little bit strange, although I'm not sure whether it's technically incorrect. "Attend" is a pretty reliable substitute. You could also say "study at university", but as a native speaker of American English, "study at…" sounds very British to me.

It's gratifying that I am still learning new things.

This is correct but sounds very formal; most people would probably say "I'm glad that…" or "I'm grateful that…".