Mense Septembri discipuli ad scholam redeunt, et September novum laborem secum fert.

They are two different forms of the same word.

• Septembri is the ablative singular
• September is the nominative singular

In mense Septembri, the word is part of a time expression, so the ablative is used.

In September novum laborem secum fert, September is the subject of fert, so it is nominative.

This is a very normal feature of Latin: the form of a noun changes depending on its job in the sentence.

Latin often uses the ablative of time when without a preposition.

So instead of needing a separate word for in, Latin can simply use the ablative case to express time.

Here:

• mense = in the month
• Septembri = of September / in September, in agreement with that expression

So mense Septembri means in the month of September, or more naturally, in September.

Mense is the ablative singular of mensis, meaning month.

So the phrase literally means:

• mense Septembri = in the month of September

Latin sometimes includes mensis / mense with month names, especially in fuller or slightly more formal phrasing. In many contexts, Septembri by itself could also express in September.

There are two clues:

1. discipuli is nominative plural
2. redeunt is third person plural

The verb ending -unt means they. Since discipuli is also plural and nominative, it matches the verb and works as the subject.

So:

• discipuli redeunt = the students return

Because ad takes the accusative case when it means movement toward a place.

So:

• ad scholam = to school

Here scholam is accusative singular.

This is different from a location expression. For example, if you wanted to say in school, Latin would usually use a different construction, such as in schola.

Redeunt is:

• present tense
• active voice
• indicative mood
• third person plural

Its dictionary form is redeo, redire, meaning to go back or to return.

So redeunt means they return.

It matches discipuli, which is plural.

Yes. This is a kind of personification, or at least a very natural figurative way of speaking.

Latin, like English, can say things such as:

• spring brings flowers
• winter brings cold
• September brings new work

So in the second clause, September is treated as the thing that brings something. Grammatically, that makes it the subject of fert.

Novum laborem is accusative singular masculine.

• laborem is the direct object of fert
• novum is an adjective describing laborem

In Latin, adjectives agree with the nouns they describe in:

• gender
• number
• case

So because laborem is masculine, singular, and accusative, novum must also be masculine, singular, and accusative.

Secum means with himself, herself, itself, or themselves, depending on the context.

It is made from:

• se = a reflexive pronoun
• cum = with

With certain pronouns, Latin attaches cum to the end instead of putting it before the pronoun:

• mecum = with me
• tecum = with you
• nobiscum = with us
• vobiscum = with you all
• secum = with himself / herself / itself / themselves

Here secum refers back to September, so the idea is September brings new work with it.

It may look unusual if you are expecting English word order, but it is perfectly normal Latin.

Latin word order is much more flexible than English because the endings show the grammatical function of the words.

Here the order helps with emphasis:

• Mense Septembri comes first to set the time
• discipuli appears before the verb, which is common
• fert comes at the end of its clause, which is also very common in Latin

So the sentence is not strange; it is just using normal Latin flexibility rather than rigid English-style order.

Yes, that would be possible.

Septembri by itself can express in September through the ablative of time when.

Adding mense makes the phrase fuller:

• Septembri = in September
• mense Septembri = in the month of September

So the version in the sentence is a slightly more expanded way of saying the same basic thing.

Because its subject, September, is singular.

Fert is the third person singular present active indicative of fero, meaning to carry or to bring.

So:

• September fert = September brings

Even though September is a month and not a person, it is still grammatically singular, so the verb must also be singular.