Toda la clase se echó a reír a carcajadas cuando mi amigo contó el chiste.

Because clase is a singular feminine noun, so todo has to agree with it:

• toda la clase = the whole class

Even though it refers to many people, the grammatical subject is still the singular noun clase, just like English the whole class.

That is also why the verb is singular:

• Toda la clase se echó...
• not Toda la clase se echaron...

Because the subject is la clase, which is grammatically singular.

Spanish often treats collective nouns this way:

• La gente llegó temprano.
• El grupo salió.
• La clase se echó a reír.

So even though many individuals are included, the grammar follows the singular noun.

Echarse a + infinitive is a common structure meaning to start/burst into doing something.

So:

• se echó a reír = started laughing / burst out laughing

It often gives a more sudden, lively feeling than a plain verb.

Other examples:

• Se echó a correr = He/She started running
• Se echaron a llorar = They burst into tears
• Me eché a temblar = I started trembling

Here, se is part of the expression echarse a + infinitive. It is not strongly reflexive in the literal English sense of oneself.

You should learn echarse a reír as a set phrase:

• echarse a reír = to burst out laughing
• echarse a llorar = to burst into tears

So yes, it uses a reflexive form, but in practice the best approach is to treat it as an idiomatic verbal expression.

Yes, you could, but the meaning and tone are slightly different.

• Toda la clase rió = The whole class laughed
• Toda la clase se echó a reír = The whole class burst out laughing / started laughing

The version with se echó a reír sounds more sudden and expressive. It suggests a clear moment when everyone began laughing.

With rió, the sentence is simpler and more neutral.

A carcajadas means loudly / heartily / in fits of laughter / with big laughs.

A carcajada is a loud laugh or guffaw. So:

• reír a carcajadas = to laugh loudly / to laugh out loud / to roar with laughter

It is a very common expression in Spanish.

Examples:

• Se rió a carcajadas. = He/She laughed loudly.
• Todos estaban riéndose a carcajadas. = Everyone was laughing their heads off.

Not really. They do different jobs:

• reír tells you the action: to laugh
• a carcajadas tells you the manner: loudly / heartily / in big bursts

So together:

• se echó a reír a carcajadas = burst out laughing loudly / burst into hearty laughter

It is a natural combination in Spanish, even if it may feel a bit more emphatic than the most literal English version.

Because contó is the preterite, which is normally used for a completed action that moves the story forward.

• cuando mi amigo contó el chiste = when my friend told the joke

This presents the joke-telling as a specific completed event, and then the class reacted.

If you said contaba, it would sound more like background or an ongoing action:

• cuando mi amigo contaba el chiste...

That is less natural here unless you want to emphasize that the laughing happened while he was in the middle of telling it.

So in this sentence, contó is the normal choice.

Because cuando can be followed by different tenses depending on the meaning.

Here it refers to a specific event in the past, so the preterite is natural:

• cuando mi amigo contó el chiste

That means when my friend told the joke on that particular occasion.

Compare:

• Cuando llegó, todos se callaron. = When he arrived, everyone fell silent.
• Cuando llegaba, todos se callaban. = Whenever he was arriving / When he used to arrive, everyone would go quiet.

So cuando + preterite often points to a one-time completed event.

Because these are third-person singular preterite forms, and in Spanish they carry a written accent:

• echó = he/she/it threw / started
• contó = he/she/it told

The accent matters because it can distinguish forms:

• echo = I throw / I cast, or I pour

• echó = he/she threw

• conto is not the normal past form
• contó = he/she told

So the accents are essential for correct spelling and sometimes for clarity.

Here, la clase means the class as a group of students, not the lesson itself.

So:

• Toda la clase se echó a reír = The whole class burst out laughing

In context, Spanish often uses la clase this way, just like English does.

Both are possible, but they suggest slightly different things.

• el chiste = the joke
• un chiste = a joke

Using el chiste often implies a specific joke that the speaker and listener either know about or are treating as a definite event in the story.

Spanish frequently uses the definite article in places where English might also use the, especially when referring to a specific known situation in the narrative.

So this sentence sounds like:

• my friend told the joke (the one in question / the joke he told at that moment)

If you said un chiste, it would simply mean he told a joke, with a slightly less specific feel.

Yes, it sounds very natural in Spanish from Spain.

A few especially natural parts are:

• se echó a reír — very common and idiomatic
• a carcajadas — also very common
• chiste — the normal word for joke in Spain

So as a whole, this is a very idiomatic, natural sentence.

Yes. Ponerse a + infinitive also means to start doing something.

So:

• Toda la clase se echó a reír
• Toda la clase se puso a reír

Both are correct and natural.

The difference is small, but often:

• echarse a reír can sound a bit more sudden or vivid
• ponerse a reír can sound a bit more neutral

In many everyday contexts, they are very close in meaning.

In this sentence, the structure is echarse a reír, so the infinitive is reír, not reírse.

That is because the pronominal part belongs to echarse, not to reír.

So the structure is:

• echarse a reír
• not echarse a reírse

That would be incorrect.

This is a useful thing to notice, because Spanish has many multi-word verb patterns where the pronoun belongs to the first verb, not the second one.