Learn French: En septembre, l'école commence. - In September, school starts.

In French, the names of months and days of the week are never capitalized unless they are the very first word of a sentence. So, it is septembre, not Septembre.

How do I know to use en instead of dans for the word "in" here?

When you are talking about months of the year (like septembre) or specific years, you always use the preposition en to mean "in".

Why is it l'école instead of just école? The English translation doesn't say "the school".

French almost always requires an article (like "the" or "a") before a noun. Even when talking about school in a general sense, where English drops the word "the", French still requires it. Here, la becomes l' because école starts with a vowel sound.

Is commence related to the English word "commence"?

Yes! They share the same origin. However, while "commence" sounds very formal in English, commencer is the everyday, standard French verb for "to start" or "to begin". It ends in -e here because it matches the singular subject l'école (it).