Às vezes, a brincadeira faz muito barulho e a professora pede silêncio.

Às vezes means “sometimes”, literally “at the times”.

The accent in às is the grave accent (crase) and shows that two things merged:

• preposition a (to/at)
• feminine plural article as (the)

a + as → às

So às vezes is literally “at the times”, but as an expression it simply means sometimes.

Portuguese thinks of this as “at (some) times”, so it uses the plural vezes (“times”).

You can see the same idea in English if you expand it:

• “at times”
• “at certain times”

So às vezes = “at (those) times” → “sometimes”.

Portuguese uses definite articles much more than English:

• a brincadeira = “the play / the playing / the fun”
• a professora = “the teacher”

Here we are talking about the specific play (the children’s play in class) and the specific teacher, so the article a is natural.

But with abstract or uncountable nouns like silêncio, Portuguese often omits the article when it means “some … / (a bit of) … in general”:

• pede silêncio ≈ “asks for silence (in general)”
• pede o silêncio would sound more like “asks for the silence” (more specific or emphatic)

So:

• definite, concrete things → usually with article
• abstract “stuff” like paz, silêncio, atenção → article often omitted when you just want “some X”.

Brincadeira comes from brincar (“to play”). It can mean:

• children’s play / playing together
• fun, messing around
• a joke / prank, depending on context

In this sentence it refers to the children playing / the playful activity that is making a lot of noise, not a “joke” in the adult sense.

Both are grammatically correct, but they don’t feel the same:

• fazer barulho = “to make noise / to be noisy (by producing noise)”
  
  • a brincadeira faz muito barulho = “the play makes a lot of noise”
• ser barulhento/barulhenta = “to be (a) noisy (person/thing)”
  
  • a brincadeira é muito barulhenta = “the play is very noisy”

Fazer (muito) barulho is a very common, natural collocation to talk about noise that an activity or group produces. That’s why it’s preferred in this context.

The grammatical subject is a brincadeira (singular feminine noun), so the verb must be singular: faz.

Even if many children are involved, the sentence is treating all that activity as one thing: “the play / the playing”:

• a brincadeira faz muito barulho = “the play makes a lot of noise”
• If you made the children the subject: As crianças fazem muito barulho (“The children make a lot of noise”) → plural verb fazem.

In Portuguese, pedir already includes the idea of “asking for”:

• pedir algo = “to ask for something”
• pedir silêncio = “to ask for silence”

So a professora pede silêncio literally is “the teacher asks for silence.”
You don’t need a preposition like in English; silêncio is the direct object of pede.

Yes, you can expand it in several natural ways:

• A professora pede para fazer silêncio.
  “The teacher asks (them) to be quiet.”
  Literally “asks to make silence.”

• A professora pede silêncio às crianças.
  “The teacher asks the children for silence.”

Word order:

• Normal order is Subject – Verb – Object – Indirect object
  
  • A professora pede silêncio às crianças.
• You can put the indirect object first for emphasis or style, but it’s less neutral:
  
  • Às crianças, a professora pede silêncio. (more marked, focusing on às crianças)

When you talk about a specific teacher as the subject of a sentence, European Portuguese almost always uses the article:

• A professora chegou. = “The teacher arrived.”
• A professora pede silêncio. = “The teacher asks for silence.”

You usually omit the article with professions only in identification sentences:

• Ela é professora. = “She is a teacher.”

So:

• A professora (subject, specific person) → with article
• Ela é professora (her job) → without article

Portuguese normally marks the gender of professions:

• o professor = male teacher
• a professora = female teacher

Since the sentence refers to a female teacher, it uses a professora.
The article a (feminine) matches the feminine noun ending -a.

Às vezes is a time expression (an adverbial phrase). When you place it at the beginning of the sentence, it is standard to separate it with a comma:

• Às vezes, a brincadeira faz muito barulho…
• If you move it, you normally drop the comma: A brincadeira, às vezes, faz muito barulho. (commas optional and more stylistic here)

So the comma just marks that Às vezes is an introductory element, like in English:
“Sometimes, the play makes a lot of noise…”

Here, the present tense in Portuguese expresses a habitual/general situation, just like English:

• Às vezes, a brincadeira faz muito barulho e a professora pede silêncio.
  = “Sometimes, the play makes a lot of noise and the teacher asks for silence.” (on different occasions)

It does not mean a specific moment right now; it describes what typically happens in that classroom.