Кое-кто из студентов уже сдал реферат, а кое-кто ещё пишет черновой вариант.

Here кое-кто means some people, certain people, or more naturally in this sentence, some of the students.

A useful nuance:

• кто-то = someone / somebody, usually neutral and often truly unknown
• кое-кто = someone / certain people, often with the idea that the speaker either knows who they mean or does not want to name them directly

So кое-кто из студентов is basically some of the students or certain students.

Because кое-кто is grammatically singular in Russian, even when English would naturally translate it as some people or some of the students.

That is why the sentence has:

• кое-кто ... сдал
• кое-кто ... пишет

not plural forms like сдали or пишут.

So the idea is plural in meaning, but singular in grammar.

There are two things happening here:

1. Not сдали
   As above, кое-кто is grammatically singular, so the verb must also be singular.

2. Why masculine singular?
   In the past tense, Russian marks gender in the singular:

   • сдал = masculine
   • сдала = feminine
   • сдало = neuter

When the person is unspecified or the group is mixed/unknown, Russian often uses masculine singular as the default form. So кое-кто ... сдал is normal here.

After из, Russian normally uses the genitive case.

So:

• из студентов = from among the students / of the students

This structure is very common when you mean some of a group:

• кто-то из друзей = someone from among the friends
• некоторые из книг = some of the books
• кое-кто из студентов = some of the students

Here студентов is genitive plural.

Not necessarily.

The form студенты / студентов can mean:

• a group of male students, or
• a mixed group of students in general

If the group were specifically all female, Russian would usually say студентки / студенток.

So in this sentence, из студентов most naturally means from among the students, without forcing a male-only interpretation.

Here it means to submit or hand in the paper.

That is important because сдать has several meanings depending on context. For example:

• сдать экзамен = to pass an exam
• сдать отчёт / реферат / домашнее задание = to hand in, submit

So уже сдал реферат means has already handed in the paper.

This is a classic aspect choice in Russian.

• сдал is perfective past
• пишет is imperfective present

Why?

• сдал shows a completed result: the paper has already been handed in.
• пишет shows an ongoing process: the student is still working on the draft.

So the contrast is:

• some students have already finished that stage
• others are still in the middle of writing

Because сдал focuses on the completed action and its result.

• сдал = handed it in, finished that action
• сдавал = was handing it in, used to hand it in, or handed it in in a process-oriented/contextual way

With уже, Russian very often uses a perfective verb when the point is already completed:

• уже сделал
• уже написал
• уже сдал

So уже сдал реферат is exactly what you would expect.

Here ещё means still.

So:

• уже сдал = has already handed in
• ещё пишет = is still writing

This is a very common contrast in Russian:

• уже = already
• ещё = still

In other contexts, ещё can also mean more or another, but not here.

А often means more than simple and. It usually introduces a contrast, comparison, or shift.

In this sentence it is close to:

• while
• whereas
• and on the other hand

So the sentence contrasts two groups:

• some students have already submitted the paper,
• while others are still writing a draft.

Using и would sound less contrastive. А is the natural choice here.

Because Russian often uses this pattern to contrast two unspecified subgroups:

• кое-кто ..., а кое-кто ...

This is very similar to English:

• some ..., while others ...

The repetition helps show that we are talking about different students in the same larger group.

A possible alternative would be:

• Одни студенты уже сдали реферат, а другие ещё пишут черновой вариант.

But кое-кто ... а кое-кто ... sounds natural and idiomatic.

It is the direct object of пишет, so it is in the accusative case.

However, for inanimate masculine singular nouns, the accusative looks exactly like the nominative. So:

• nominative: черновой вариант
• accusative: черновой вариант

That is why it appears unchanged.

The same thing happens with реферат after сдал:

• сдал реферат

It is also accusative, but the form looks the same because the noun is inanimate masculine singular.

Because the sentence contains two coordinated clauses, and Russian normally puts a comma before а in this kind of structure:

• Кое-кто из студентов уже сдал реферат, а кое-кто ещё пишет черновой вариант.

The comma marks the boundary between:

1. Кое-кто из студентов уже сдал реферат
2. а кое-кто ещё пишет черновой вариант

This is standard Russian punctuation.