Learn Hardcore Japanese: gakuseizenin ha asita no siken no zyunbi wo simasu. - All the students will prepare for tomorrow’s exam.

学生全員は… = introduces “all the students” as the topic; often contrasts them with something else or sets the scene.
学生全員が… = marks “all the students” more straightforwardly as the grammatical subject, often used in neutral descriptions or when answering “who will do it?”.

In many real contexts:

学生全員は〜 can sound a bit like: “All the students (as opposed to others) will prepare …”
学生全員が〜 might sound more like: “It is all the students who will prepare …” (answering a “who?” question).

What exactly does 学生全員 mean? Is it plural? How is it different from 全ての学生?

学生全員 literally means “the whole membership of students” → “all the students”.

学生 = student / students (no plural marker)
全員 = everyone / all members

Japanese usually doesn’t mark plural on nouns the way English does, so 学生 can mean student or students depending on context; 全員 makes it clear we mean “all of them”.

学生全員 and 全ての学生 both mean “all the students”, and in many situations they are interchangeable.

Nuance:

学生全員 feels a bit like “the entire group of students”.
全ての学生 is more literally “every student / all students” and is a bit more formal or written-sounding.

In this sentence, both:

学生全員は明日の試験の準備をします。
全ての学生は明日の試験の準備をします。
are natural and have essentially the same meaning.

There is no word like “will” in the Japanese, so why does します mean “will do / will prepare” here?

Japanese non-past verb forms (dictionary form or polite 〜ます form) cover both:

present / habitual
and
future

します is the polite non-past form of する (“to do”). Depending on context, it can mean:

“do / does”
“will do”
“is going to do”

Here, with 明日 (“tomorrow”), the time is clearly in the future, so します is naturally understood as “will do (prepare)”.

So:

学生全員は明日の試験の準備をします。
= “All the students will prepare for tomorrow’s exam.”
(no extra word for “will” is needed in Japanese)

Why are there two の in a row: 明日の試験の準備? How does this structure work?

Each の here is linking nouns together into a bigger noun phrase.

Break it down:

明日の試験
- 明日 = tomorrow
- 試験 = exam/test
- 明日の試験 = “tomorrow’s exam”
明日の試験の準備
- 準備 = preparation
- (明日の試験) の準備 = “preparation for tomorrow’s exam”

So the full block:

明日の試験の準備 = “the preparation for tomorrow’s exam”

In English we use many different patterns:
tomorrow’s exam, preparation for the exam, exam preparation, etc.
In Japanese, の is very flexible and can cover relations like:

possession: 田中さんの本 (Tanaka’s book)
“of”: 日本の歴史 (history of Japan)
“for / about”: 試験の準備 (preparation for the exam)

So the two の are just chaining the nouns: 明日 → 試験 → 準備.

Why is 明日 followed by の instead of something like に? When do we use に with time expressions?

Here, 明日 is not being used as a time adverb (“tomorrow (at that time)”), but as a modifier of 試験:

明日の試験 = “tomorrow’s exam”
→ の is correct because 明日 is describing the kind of 試験.

If you use 明日 to say “do (something) tomorrow” (adverbial time expression), you normally don’t use any particle:

明日試験があります。 = “There is an exam tomorrow.”
明日勉強します。 = “I will study tomorrow.”

You can sometimes use に with time words:

日曜日に行きます。 = “I’ll go on Sunday.”
３時に会いましょう。 = “Let’s meet at 3 o’clock.”

But with common adverbial time words like 今日, 明日, 昨日, the particle is often dropped in everyday speech:

明日行きます。 (more common than 明日に行きます, which usually sounds odd or overly specific).

In your sentence, 明日 is part of the noun phrase 明日の試験 (“tomorrow’s exam”), not a standalone time adverb, so の is required.

What is the difference between 試験の準備をします and 試験を準備します?

試験の準備をします is natural and means
“(someone) will do the preparation for the exam” / “will prepare for the exam.”
試験を準備します is grammatically possible but usually unnatural in this context. It literally sounds more like:
- “(someone) will prepare the exam (itself)”
  (e.g., a teacher making the test questions)

Why:

準備 = “preparation” (a noun)
〜の準備をする = “do the preparation of/for 〜” → idiomatic way to say “prepare for 〜”

So for a student getting ready to take an exam, you say:

試験の準備をする / 試験の準備をします
not 試験を準備する.

Why do we say 準備をします instead of just 準備します? Are both correct?

Both forms exist:

準備をします
- Literally: “do preparation”
- 準備 (noun) + を (object marker) + します (do)
- Very clear and standard.
準備します
- This is 準備をします with を dropped.
- In everyday speech, it’s common to omit を after some Sino-Japanese nouns like 準備:
  - 勉強(を)します (study)
  - 運動(を)します (exercise)
  - 練習(を)します (practice)

So you could also say:

学生全員は明日の試験の準備します。

However, in careful or textbook-style writing, 準備をします is often preferred because it’s explicit and clearly shows the object with を. In polite, formal sentences like this, 準備をします sounds slightly more complete and standard.

Could the word order be different? For example, can I move 明日 or 学生全員 to other positions?

Japanese word order is relatively flexible as long as the verb comes at the end and the particles stay attached to the right words.

Your original:

学生全員は明日の試験の準備をします。

Other natural variants:

明日、学生全員は試験の準備をします。
(puts emphasis on “tomorrow” at the start)
学生全員は試験の準備を明日します。
(slight emphasis on “tomorrow” as when the preparation will be done)
明日学生全員は試験の準備をします。
(comma optional in speech; still natural)

Less natural but still possible:

試験の準備を学生全員は明日します。
(now the topic 学生全員は floats in the middle; can sound marked or contrastive)

The safest, most neutral for learners is something close to the original:
[topic] は [details] をします.

What politeness level is します, and how would this sentence look in casual speech?

します is the polite (丁寧語) non-past form of する.

Politeness level:

Used in most public situations, with teachers, in class, with people you’re not close to, etc.
Neutral polite, not especially formal/honorific, just standard politeness.

In casual/informal speech, you’d use する instead of します:

学生全員は明日の試験の準備をする。

If everyone already knows you’re talking about the students, you might also drop the topic:

明日の試験の準備をする。 (“[We/They/I] will prepare for tomorrow’s exam.”)

Why is を attached to 準備 and not to 試験?

を marks the direct object of the verb.

In this sentence, the main verb is:

します = “do”

The thing being “done” is:

準備 = preparation
more specifically, 明日の試験の準備 = “preparation for tomorrow’s exam”.

So:

明日の試験の準備をします。
→ “(They) will do the preparation for tomorrow’s exam.”

試験 is not the direct object of します here; it’s inside the noun phrase 試験の準備 (“preparation for the exam”). So を attaches to the whole noun phrase head 準備, not to 試験.

Could this sentence mean “I will prepare all the students for tomorrow’s exam”? Who is the subject?

No, in normal interpretation it does not mean “I will prepare all the students”. It means:

“All the students will prepare (for tomorrow’s exam).”

Subject/topic:

学生全員は is the topic (and effectively the subject) of します.
Japanese often omits explicit pronouns like “I”, “we”, “they”.
Here, the default reading is: 学生全員 are the ones who します (do the preparation).

To express “I will prepare all the students for tomorrow’s exam”, you’d need a different structure, for example:

私は学生全員を明日の試験のために指導します。
(“I will train/coach all the students for tomorrow’s exam.”)
私は学生全員を明日の試験に備えさせます。
(“I will make/have all the students prepare for tomorrow’s exam.”)

So in the given sentence, 学生全員 are the people doing the preparation, not the ones being prepared by someone else.