Learn Hardcore Japanese: sensei ha gakusei ni zyugyou no kansou wo kikimasita. - The teacher asked the students for their impressions of the class.

You could use 先生が instead. Then 先生 is emphasized more as the subject that performed the action, often in contrast to someone else (e.g. “It was the teacher (not someone else) who asked…”).

So:

先生は学生に授業の感想を聞きました。 – Neutral “The teacher asked the students…”
先生が学生に授業の感想を聞きました。 – Focus on the teacher as the doer, useful in contrastive contexts.

Why is 学生 marked with に instead of を?

With the verb 聞く, when it means “to ask (someone)”, the person you ask is marked with に, and the thing you ask about is marked with を.

Pattern: [person] に [thing] を聞く

So in this sentence:

学生に – “to the student(s)” (the person being asked)
授業の感想を – “the impressions of the class” (what is being asked for)

If you used 学生を, it would suggest the students themselves are the direct object (which doesn’t fit with this meaning of 聞く).

What does 授業の感想 literally mean, and how does の work here?

授業の感想 literally is:

授業 – class, lesson
の – “of”, showing a close relationship
感想 – impressions, thoughts, feedback, what you felt/thought about something

So 授業の感想 = “impressions of the class”, “thoughts about the class”, “feedback on the lesson”.

Here の connects two nouns, where the first noun (授業) modifies the second (感想), similar to an “X of Y” or “Y’s X” structure in English.

Where is the English “about” in “asked (the students) about the class” in this Japanese sentence?

The idea of “about” is built into the phrase 授業の感想.

Rather than saying “asked about the class” directly, Japanese often says “asked (for) the impressions of the class”:

English: “asked the students about the class”
Japanese: 学生に授業の感想を聞きました – literally: “asked the students for the class’s impressions”

So の in 授業の感想 covers what English expresses with “about the class”.

Why is 感想 marked with を?

感想 is the direct object of the verb 聞きました in this sentence.

With this meaning of 聞く = “to ask (for) / to inquire about”, the thing you ask for is marked by を:

感想を聞く – “ask for (someone’s) impressions”
道を聞く – “ask for directions”
理由を聞く – “ask (someone) the reason”

So 授業の感想を聞きました = “(someone) asked for the impressions of the class.”

Does 聞きました here mean “asked” or “listened”?

It means “asked” here.

聞く has two common meanings:

“To hear / listen (to)” – e.g. 音楽を聞く (listen to music)
“To ask / inquire” – e.g. 先生に質問を聞く (ask the teacher a question)

When the pattern is [person] に [information/thing] を聞く, it almost always means “to ask (someone) [something]”.
So 学生に授業の感想を聞きました is understood as “(The teacher) asked the students for their impressions of the class,” not “listened to the students’ impressions”.

Why is 聞く changed to 聞きました and not 聞くました?

To make the polite ます-form, you first make the ます-stem of the verb, then add ます (and then conjugate that for tense).

For 聞く:

Dictionary form: 聞く
ます-stem: 聞き (the く changes to き)
Polite non-past: 聞きます
Polite past: 聞きました

So 聞きました is simply the polite past form of 聞く. Japanese never says 聞くました.

What tense and politeness level is 聞きました, and how would this change in casual speech?

聞きました is polite past (polite version of “asked”).

Polite non-past: 聞きます – “ask / will ask”
Polite past: 聞きました – “asked”

In casual speech, you usually use the plain (dictionary) forms:

Casual non-past: 聞く
Casual past: 聞いた

So a casual version might be:

先生は学生に授業の感想を聞いた。 – “The teacher asked the student(s) for their impressions of the class.” (casual)

Can the word order be changed, or must everything stay exactly in this order?

Japanese word order is relatively flexible as long as the particles stay attached to the right words. The verb must come at the end, but the parts before it can usually be shuffled.

For example, all of these are natural:

先生は学生に授業の感想を聞きました。
先生は授業の感想を学生に聞きました。
学生に先生は授業の感想を聞きました。

The nuance can shift slightly depending on what you put early for emphasis or topic flow, but grammatically they are fine. The key point is that は / に / の / を stay attached to the same nouns.

Could 学生に授業の感想を聞きました。 be a complete sentence without 先生は?

Yes. Japanese often omits the subject when it is clear from context.

学生に授業の感想を聞きました。
In many situations, this will naturally be understood as “I asked the students for their impressions of the class” if you are talking about yourself.

Adding 先生は just makes the subject explicit and makes it clear that we are talking about “the teacher” (and not “I” or someone else). Context decides which is needed.

Is 先生 always “teacher”? Could it refer to someone else?

先生 often means “teacher”, but it has a broader use:

School/lesson context: “teacher”
Doctors, lawyers, politicians, and other professionals: a polite title, “Dr.”, “Professor”, “Counselor”, etc.
Artists, writers, masters of a craft: respectful title, “Master”, “Sensei”

In this particular sentence, because 授業 (class/lesson) is mentioned, 先生 is naturally interpreted as “the teacher (of the class)”.

Is there a difference between 授業の感想 and 授業についての感想?

Both can be translated as “impressions of the class”.

授業の感想
Shorter, very natural, and commonly used. It simply treats the class as the thing that the impressions are about.
授業についての感想
Literally “impressions about the class”.
について explicitly means “about / concerning”, so this can sound a bit more formal or explicit, like in surveys or written instructions.

In everyday speech about a class, 授業の感想 is perfectly natural and often preferred for its simplicity.