Nonnulli discipuli tacent, alios magistra reprehendit.

Yes—historically, that is exactly where it comes from. Nonnulli literally means not none, but in normal Latin it functions as an idiomatic way to say some or several.

So:

• nonnulli discipuli = some students
• not no students

This is a very common Latin expression.

Nonnulli discipuli is nominative plural masculine, which marks it as the subject of tacent.

You can tell this from the endings:

• nonnulli = nominative masculine plural
• discipuli = nominative masculine plural
• tacent = they are silent / they keep silent, so it needs a plural subject

So the first clause is:

• Nonnulli discipuli tacent = Some students are silent

Because its subject is plural: nonnulli discipuli = some students.

Latin verbs agree with their subject in number:

• tacet = he/she/it is silent
• tacent = they are silent

Since students is plural, Latin uses tacent.

Because alios is accusative plural, while alii would be nominative plural.

In the second clause, others is the direct object of reprehendit (scolds/rebukes), so Latin uses the accusative:

• alios = other people / others as object
• alii = other people / others as subject

So:

• magistra reprehendit alios = the teacher scolds others

If Latin had alii magistra reprehendunt, that would mean something like other people scold the teacher or other people scold, depending on the rest of the sentence—not the same idea at all.

The noun is understood from the context: discipulos.

So alios means other students here, even though Latin does not repeat the noun. This is very common.

You can think of it as:

• Nonnulli discipuli tacent, alios (discipulos) magistra reprehendit.
• Some students are silent; the teacher scolds other students.

Latin often leaves out a noun when it is obvious from the previous words.

Latin word order is much freer than English word order because the endings show the grammatical roles.

So in:

• alios magistra reprehendit

the endings tell you that:

• alios = accusative plural = direct object
• magistra = nominative singular = subject
• reprehendit = singular verb = she scolds

Putting alios first gives it emphasis or contrast:

• Some students are silent; others the teacher scolds.

That fronted alios helps highlight the contrast between some and others.

Yes, definitely.

The sentence sets up a contrast:

• Nonnulli discipuli tacent = Some students are silent
• alios magistra reprehendit = the teacher scolds others

So the idea is:

• some students are behaving quietly,
• but other students are not, and the teacher rebukes them.

Even without a word like but, the contrast is clear.

Latin does not always need a conjunction where English often uses one.

Here the comma is enough to connect the two ideas:

• Nonnulli discipuli tacent, alios magistra reprehendit.

English might translate it as:

• Some students are silent, but the teacher scolds others.
• or Some students are silent, and the teacher scolds others.

The Latin leaves the relationship slightly more open, but the contrast strongly suggests but in English.

Reprehendit means she rebukes, scolds, blames, or criticizes.

It is from reprehendere.

In a classroom context, the teacher scolds others is a very natural translation.

So:

• magistra reprehendit = the teacher scolds / rebukes

Yes. Magistra is nominative singular, so it is the subject.

Latin relies on case endings more than position:

• magistra = nominative singular = subject
• alios = accusative plural = object

So even though English usually prefers The teacher scolds others, Latin can say:

• alios magistra reprehendit

and still clearly mean:

• the teacher scolds others

They are forms of adjectives, but here they function a bit like pronouns too.

• nonnulli modifies discipuli, so it is clearly adjectival there: some students
• alios stands by itself, but it implies other students, so it is acting pronominally: others

This is very common in Latin. An adjective can be used:

• with a noun: alios discipulos = other students
• without the noun: alios = others

It has two clauses:

1. Nonnulli discipuli tacent

   • nonnulli discipuli = subject
   • tacent = verb

2. alios magistra reprehendit

   • alios = direct object
   • magistra = subject
   • reprehendit = verb

So the overall pattern is:

• Some students are silent; the teacher scolds others.

The second clause puts alios first for contrast and emphasis.