Todo el mundo se lleva bien en la clase.

Literally, todo el mundo means the whole world, but in everyday Spanish it very often means everyone or everybody, just like in this sentence.

So:

• Literal: the whole world
• Usual meaning in contexts like this: everyone (in that group / context)

Grammatically, todo el mundo is singular, even though it refers to many people. The verb agrees with the grammar, not the real number of people.

• todo el mundo → treated as he / she / it (3rd person singular)
• So we say: Todo el mundo se lleva bien (not se llevan).

• llevar (without se) most often means to carry, to take, or to wear.

  • e.g. Llevo una chaqueta. – I’m wearing a jacket.

• llevarse bien (con alguien) is a reflexive / reciprocal verb meaning to get along (with someone).

  • e.g. Me llevo bien con mis vecinos. – I get along well with my neighbours.

In your sentence, se lleva bien expresses that people get along with one another.

Here, se is part of the pronominal verb llevarse bien, which often has a reciprocal meaning: people get along with each other.

You can think of:

• Todo el mundo se lleva bien (unos con otros).
  → Everyone gets along (with one another).

Without se, llevar bien would not have this “get along” meaning.

• bien is an adverb: it describes how an action is done.
• bueno is an adjective: it describes a noun.

Here we are describing how people get along (the verb llevarse), so we need the adverb:

• se lleva bien → gets along well

You would use bueno with a noun, e.g. un buen profesor (a good teacher).

se lleva is 3rd person singular, present indicative of llevarse.

Here, the present indicates:

• a general, habitual situation: something that is generally true now, not just at this exact moment.
  So it means that, as a rule, everyone in the class gets along well.

Yes, you can say en clase (without la). Both are correct, but there is a nuance:

• en clase often means in class / during class in a general, more abstract sense.
• en la clase sounds more like in the class (group) or in that specific class.

In this sentence, en la clase suggests within that particular class group.

Yes, that word order is perfectly natural: En la clase todo el mundo se lleva bien.

Meaning doesn’t change; you just emphasize the context first:

• En la clase (in the class), todo el mundo se lleva bien (everyone gets along).

You could also say Todo el mundo en la clase se lleva bien, which is also fine.

Yes, Todos se llevan bien en la clase is correct and very common. Differences:

• Todo el mundo

  • Slightly more informal, very colloquial and frequent.
  • Grammatically singular → se lleva.

• Todos

  • Explicitly plural → requires plural verb: se llevan.
  • Sometimes feels a bit more “neutral” or less colloquial.

Meaning is essentially the same here: everyone gets along.

Todo el mundo is used in both Spain and Latin America. It’s very common everywhere in the Spanish‑speaking world.

In Spain, you’ll hear both:

• Todo el mundo se lleva bien…
• Todos se llevan bien…

On its own, Todo el mundo se lleva bien en la clase usually implies that the classmates get along with each other.

If someone wanted to be explicit about the teacher, they’d typically say:

• Todo el mundo se lleva bien con el profesor / la profesora.

Context would clarify whether the teacher is included, but the default reading is “with each other.”

Because clase is a feminine noun in Spanish.

• la clase → the class
• Adjectives and articles must match the noun’s gender and number, so you say:
  
  • la clase grande, una clase interesante, etc.