Classmates

I would change the title, since your story is really not about your classmate. Rather it is about the fragility of life.

Again, another classmate has passed away.

There were no signs, all of a suddenis happened suddenly with no signs in advance.

Or you could say it this way - with no advance notice/warning

Life is so fragile, it went and it could be extinguished in a flash.

You exist, there is a world, but you don’tThe world is there when you exist, but when you are gone, there is nothing.

At the very least, we still So for now, we are alive, t. That is everything, that is more than enough!

Again, another classmate passed away.

This is redundant, in formal writing, repeat things that imply the same thing should be removed.

There were no signs, it was all of a sudden.

This would work for a poem, and I would consider this correct.

LOur life is so fragile, it went ingoes by a flash.

You're saying here that people's lives are fragile, but you used a past tense.

If you specified that "Life" here refers to the classmate that passed, it would be "His life was so fragile, it went in a flash." and that would be correct.

Also, keep your tenses consistent for the same subject.

YWhen you exist, there is a world, but. But when you don’t, there is nothing.

I did not understand this statement at first, actually.

I just assumed what you were trying to say here. Use "when" whenever you state a change of state, or just verb being done.

At the very least, we still live, that is everything, and that is more than enough!

In comma lists, always use "___, and ____" before the last item.

Example:
I like eating, playing, and resting.

Again, another classmate passed away.

'Again, another' -- you don't really need both, but it's not serious.

There were no signs,. It happened all of a sudden.

'All of a sudden' is a good English expression, but keep in mind that it would not normally be used in writing. It's very casual.
You should use 2 sentences unless you connect them with a conjunction of some kind (or a semi-colon but that's advanced punctuation), and I can't think of one that works.

Life is so fragile, it went. It goes by in a flash.

Same problem as above. This is an error called a 'comma splice' in English grammar -- you're using a comma instead of a period to separate your sentences.
In the 2nd sentence, you either mean your classmate's life went by in a flash -- in which case 'went' is correct -- or that life in general goes by in a flash -- and then you don't want the past tense.

You exist, t. There is a world, but. Then you don’t, t. There is nothing.

More of the same. I would write this using semicolons and/or dashes, but correct use of those punctuation marks is not well understood even by most English speakers:

'You exist; there is a world. Then you don't; there is nothing.'

But your parallel structure -- that is, the way the 2 parts of your first sentence (You exist; there is a world) and the second sentence (Then you don't; there is nothing) match in meaning and order -- that's very good.

At the very least, we are still alive,; that is everything, tha. It is more than enough!

Nice use of expressions. And a nice conclusion.