La profesora regaña a los más traviesos cuando hablan en voz alta en clase.

That a is the personal a. In Spanish, when the direct object is a person or people, you usually put a before it.

So in this sentence:

• La profesora regaña a los más traviesos
• literally: The teacher scolds the naughtiest ones

Even though los más traviesos is the direct object, it refers to people, so Spanish uses a.

You would not use this a with most non-human objects.

Because Spanish normally uses the definite article with this kind of superlative idea.

• los más traviesos = the naughtiest ones / the most mischievous ones

If you only said más traviesos, it would usually sound incomplete here, because it would mean more mischievous, not the most mischievous.

So:

• más traviesos = more mischievous
• los más traviesos = the most mischievous / the naughtiest ones

Because it refers to a group of people, and Spanish uses:

• masculine plural for a group of males or a mixed group
• feminine plural only for a group of females

So los más traviesos could mean:

• the naughty boys
• the naughty children
• the naughty students

If the group were definitely all female, it would be:

• las más traviesas

Yes, you can think of it that way. Spanish often leaves out a noun when it is obvious from context.

So los más traviesos really means something like:

• the naughtiest ones
• the most mischievous students
• the most mischievous children

The noun is understood, so it does not have to be said.

Because the subject of hablan is los más traviesos, which is plural.

Structure:

• La profesora regaña...
• ...a los más traviesos...
• ...cuando hablan en voz alta en clase.

So the people who speak loudly are los más traviesos, not la profesora.

That is why the verb is plural:

• hablan = they speak

Because Spanish often leaves out subject pronouns when the verb ending already shows who the subject is.

• hablan already means they speak
• so ellos is unnecessary unless you want emphasis or contrast

That is very normal in Spanish.

Here it most naturally means whenever they speak or when they speak in a habitual sense.

The whole sentence describes something that happens regularly:

• The teacher scolds the naughtiest ones when/whenever they speak loudly in class.

Spanish often uses the present tense for habitual actions like this.

If it were about one specific future event, Spanish would usually change the structure.

It is a fixed expression meaning:

• out loud
• in a loud voice
• loudly

Literally, it is:

• en = in
• voz = voice
• alta = high/loud

So hablar en voz alta is a very common way to say to speak out loud.

Spanish does not normally use an adverb equivalent to loudly here in the same way English does. En voz alta is the natural expression.

En clase is a common idiomatic expression meaning:

• in class
• during class

It refers more to the situation or activity of class time.

By contrast, en la clase often sounds more like:

• in the classroom
• in that specific class

So here en clase is the most natural choice because the idea is during class.

Regaña is the third-person singular present indicative of regañar.

• yo regaño
• tú regañas
• él/ella regaña

The present tense is used because the sentence describes a usual or repeated action:

• The teacher scolds... when they speak loudly in class.

This is not necessarily happening right now. It is more like a general fact or routine.

It usually means to scold, to tell off, or to reprimand. It is stronger than simply to correct, but not necessarily extremely harsh.

In a school context, la profesora regaña suggests that the teacher tells the misbehaving students off for what they are doing.

So it is a very natural verb here.

The sentence follows a very normal Spanish pattern:

• La profesora = subject
• regaña = verb
• a los más traviesos = direct object
• cuando hablan en voz alta en clase = time clause

So the order is basically:

• The teacher scolds the naughtiest ones when they speak loudly in class.

Spanish can change word order for emphasis, but this version is neutral and natural.