Proszę zachowywać się ciszej podczas lekcji.

Proszę here is a polite way to say please when making a request.

In Polish, proszę + infinitive is a very common formal or polite structure:

• Proszę usiąść. = Please sit down.
• Proszę czekać. = Please wait.
• Proszę zachowywać się ciszej. = Please behave more quietly / Please be quieter.

So in this sentence, proszę does not mean I ask literally in the way English would usually phrase it. It functions as a polite request.

Zachowywać się means to behave.

It is made of:

• zachowywać = to behave / to act
• się = a reflexive particle that belongs with the verb

You should learn it as a whole expression: zachowywać się.

Examples:

• On zachowuje się grzecznie. = He behaves politely.
• Dlaczego tak się zachowujesz? = Why are you behaving like that?

In your sentence, zachowywać się ciszej means something like to behave more quietly or to act more quietly.

After proszę, Polish very often uses the infinitive.

So instead of saying:

• Zachowujcie się ciszej! = Be quieter! / Behave more quietly!

you can say:

• Proszę zachowywać się ciszej. = Please behave more quietly.

The version with proszę + infinitive sounds more polite, less direct, and often more formal. It is common in schools, offices, public announcements, and instructions.

This is about aspect.

• zachowywać się = imperfective
• zachować się = perfective

In polite requests like this, the imperfective is very common because it focuses on the ongoing manner of behavior:

• Proszę zachowywać się ciszej. = Please behave more quietly.

It suggests a continuing action during the lesson, not just one single completed act.

If you used zachować się, it would sound more like a single completed behavior in a particular moment, which is less natural here.

Się is a clitic-like particle in Polish, and its position is flexible, but not completely random.

Here:

• Proszę zachowywać się ciszej.

This is a natural word order. With infinitives, się often comes after the infinitive:

• ubierać się = to get dressed
• śmiać się = to laugh
• zachowywać się = to behave

So the placement here is normal and idiomatic.

Ciszej is the comparative form of cicho.

• cicho = quietly
• ciszej = more quietly / quieter

So:

• Mów cicho. = Speak quietly.
• Mów ciszej. = Speak more quietly / Speak quieter.

In your sentence, the speaker is asking for a reduction in noise, so ciszej is the natural choice:

• Proszę zachowywać się ciszej. = Please behave more quietly.

Here it is an adverb, because it describes how someone should behave.

Compare:

• cichy uczeń = a quiet student → adjective
• uczeń zachowuje się cicho / ciszej = the student behaves quietly / more quietly → adverb

Since it modifies the verb phrase zachowywać się, it must be adverbial.

Podczas means during, and it requires the genitive case.

So:

• lekcja = lesson
• genitive singular: lekcji

That is why you get:

• podczas lekcji = during the lesson

More examples:

• podczas obiadu = during dinner
• podczas spotkania = during the meeting
• podczas wakacji = during the holidays

Yes, often you can.

• podczas lekcji = during the lesson
• na lekcji = in class / during class

Both are possible, but they are not always identical in feel:

• podczas lekcji sounds a bit more formal and emphasizes the time period
• na lekcji is very common in everyday speech

So you might also hear:

• Proszę zachowywać się ciszej na lekcji.

That is perfectly natural too.

This sentence does not directly show singular or plural, because the structure proszę + infinitive avoids personal verb endings.

So:

• Proszę zachowywać się ciszej. can be said to one person politely
• it can also be said to a group

The exact meaning depends on context.

That is one reason this structure is useful: it is polite and somewhat impersonal.

Yes. It sounds natural, polite, and slightly formal.

A teacher might say it to students, especially in a formal or controlled tone. Depending on the situation, other natural classroom alternatives include:

• Proszę być ciszej.
• Proszę o ciszę.
• Zachowujcie się ciszej.

Your sentence is correct and understandable, though Proszę o ciszę or Proszę zachowywać się ciszej may sound a bit more official than casual everyday teacher talk.

A more direct version would use the imperative:

• Zachowuj się ciszej. = Be quieter. / Behave more quietly.
  (to one person, informal)

• Zachowujcie się ciszej. = Be quieter. / Behave more quietly.
  (to more than one person)

The original:

• Proszę zachowywać się ciszej podczas lekcji.

is more polite, more formal, and less personal.