There were more than 200 people ( that was surprising), and we enjoyed talking with our old friends, especially our classmates.

Of course, it is wonderful that she got married and had a child, but she was the most popular teacher during our school days, so i. It seems sad that no morew students will not be taught by her.

The "," after "Of course" is techinically optional, but the sentence flows a little bit better if you remove it.

I split the sentence into two, but your original sentence is joined together correct and this is more of a writing style preference!

Anyway, it was a special time thatwhere I met old friends and rememberedinisced about our nostalgic days.

"That I met old friends" technically works, but it sounds odd. "Where" here is talking about a place in time where you were meeting with your old friends.

I changed "remembered" to "reminisced" as it gives more context of "longing" or "happily recalling" of past events.

I was shocked to hear that my homeroom teacher had already quit her job to take care of her baby.

Anyway, it was a special time that I metI had a great time meeting old friends and remembereding our nostalgic days.

I feel using the 'ing form' is a little more natural! And you typically say "have a great time" - again just for more natural use!

The other day, I went to my high school reunion party.

"Reunion" will suffice here to explain what kind of gathering it is. To say "reunion party" is redundant.

I met up with my friend at the train station, then we walked to the hall together.

I'm assuming it was the train station but you'll want to be clear because there are other types of stations in English.

There were more than 200 people ( that was surprising), and we enjoyed talking with our old friendclassmates, especially our classmateold friends.

It makes sense to generalize that you talked to your old classmates and then specify with "especially" when it comes to your old friends.

Of course, it is wonderful that she got married and had a child, but she was the most popular teacher during our school days, so it seem's sad that no more students will be taught by her.she won't teach any more students.

Your phrasing here isn't incorrect ("that no more students will be taught by her") but it's in the passive voice, so that's a bit discouraged in writing. It sounds better to say "that she won't teach any more students".

Anyway, it was a special time that I met up with old friends and remembered our nostalgic days.

"Met old friends" makes it seem like you are just meeting them for the first time, which doesn't make sense.
"Met up with old friends" refers more to hanging out with them in the current moment.

How often do you get such athe chance to gather with your classmates after graduation?

There were more than 200 people ( that was surprising), and we enjoyed talking with our old friends, especially our classmates.

Some of our old homeroom teachers also came.

Adding "old" here makes it clear that they are not still your homeroom teachers

AnywayIn any case, it was a special time that- I met up with my old friends and we remembered our nostalgicinisced on our high school days.

I have tried to make this both a bit more natural and a bit more advanced for you

How often do you get such athe chance to gather with your classmates after graduation?