Learn Hardcore Norwegian: De snakker om en skole.

Skole is a masculine noun in Norwegian. Masculine nouns take the indefinite article en, while neuter nouns take et (for example, et hus) and feminine nouns often take ei (although many speakers simply use en for feminine nouns too).

What does "De" mean here, and does the capital "D" have any special significance?

In this sentence, De simply means they. The capital letter at the start is because De starts the sentence. It is not the formal/polite "you" as in some languages (like German’s Sie). Nor is it the older, now quite rare, polite form found sometimes in older Norwegian texts.

Is the word order "De snakker om en skole" typical for Norwegian, and can it be changed?

Yes, this word order (Subject + Verb + (adverb/preposition) + Object) is typical in Norwegian. You could say something like De snakker om den nye skolen ("They talk about the new school") by substituting en skole with den nye skolen, but the core word order remains the same.

How would I make it past tense, like "They talked about a school"?

To make it past tense, you change snakker (present tense) to snakket (simple past):
• De snakket om en skole.

Can "om" mean something else, or is it always translated as "about"?