W tej klasie uczy się dwadzieścia dzieci, ale dziś dwie osoby są chore.

Because after w meaning in, Polish uses the locative case.

• basic form: ta klasa = this class
• after w: w tej klasie = in this class

Both words change because Polish adjectives/demonstratives and nouns have to match in case, gender, and number:

• ta → tej
• klasa → klasie

So w tej klasie is the correct locative form.

Here uczy się comes from the verb uczyć się, which means to learn or to study.

This is different from:

• uczyć kogoś = to teach someone
• uczyć się = to learn / study

So:

• Dzieci uczą się = The children study / learn
• Nauczyciel uczy dzieci = The teacher teaches the children

In other words, się is part of the verb uczyć się. It does not literally mean that the children are teaching themselves in the English sense.

This is a very important Polish grammar pattern.

With numerals 5 and above, Polish often treats the subject as a special numeral phrase, and the verb is commonly in the 3rd person singular rather than normal plural agreement.

So:

• dwie osoby są chore = plural agreement
• dwadzieścia dzieci uczy się = singular agreement

You can see this even more clearly in the past tense:

• Dwadzieścia dzieci uczyło się
  not uczyły się

This is normal Polish grammar with larger numbers.

A useful shortcut:

• 2, 3, 4 → usually plural agreement
• 5+ → often singular verb in these counting constructions

Because after numerals like pięć, sześć, dwadzieścia, Polish normally uses the genitive plural form of the noun.

The singular noun is:

• dziecko = child

Its plural is irregular:

• dzieci = children

And its genitive plural is also dzieci, so after dwadzieścia you get:

• dwadzieścia dzieci

That may look easy on the surface, but it is actually an irregular noun.

Compare:

• dwa dziecka ❌
• dwadzieścia dziecków ❌
• dwadzieścia dzieci ✅

Because osoba is a feminine noun, and the numerals 2, 3, 4 change for gender.

For 2:

• dwaj = masculine personal
• dwa = neuter / some masculine non-personal
• dwie = feminine

Since osoba is feminine, you must say:

• dwie osoby = two people

So even if the two people are children of any sex, the grammar follows the noun osoby, and osoba is feminine.

Because these are two different number patterns in Polish.

With 2, 3, 4

The phrase behaves more like a normal plural subject:

• dwie osoby są chore

With 5 and above

The phrase behaves like a numeral construction that often takes singular verb agreement:

• dwadzieścia dzieci uczy się

So the contrast is completely normal:

• 2 people are sick → plural
• 20 children study → singular-style numeral agreement

This is one of the trickiest features of Polish numbers, and learners notice it very quickly.

Because chore agrees with osoby, which is a non-masculine-personal plural form.

In Polish plural adjective forms, there is an important split:

• chorzy = masculine personal plural
• chore = non-masculine-personal plural

Since osoby is grammatically feminine plural, the adjective must be:

• dwie osoby są chore

Compare:

• dwaj chłopcy są chorzy = two boys are sick
• dwie osoby są chore = two people are sick

Even if those osoby are male in real life, the adjective still agrees with the grammatical gender of osoba.

This is mostly a matter of natural style.

Using osoby:

• avoids repeating dzieci
• focuses on two individuals
• sounds quite natural in Polish

So dwie osoby są chore is like saying two people are sick.

It does not mean they are not children; it just uses a more general word in the second clause.

Polish word order is fairly flexible, so dziś could move.

The sentence given is:

• W tej klasie uczy się dwadzieścia dzieci, ale dziś dwie osoby są chore.

This placement highlights today as a contrast:

• usually there are twenty children studying in the class,
• but today two are sick.

Other word orders are possible, for example:

• Ale dwie osoby są dziś chore.

That means almost the same thing, but the emphasis shifts slightly. Polish often uses word order for emphasis rather than strict grammatical necessity.

In principle, klasa can mean either:

• a classroom
• a class/group of students

But in this sentence, it almost certainly means the class as a group, because of the idea that twenty children study in this class.

So the sentence is talking about the membership of the class, not mainly about how many children are physically inside a room at one moment.

Yes. Putting W tej klasie at the beginning sets the scene first:

• As for this class...
• In this class...

This is very natural Polish word order. It tells you the context before giving the information about how many children study there.

A more neutral-looking order might be possible in other contexts, but starting with the place/topic is extremely common in Polish.