Learn Spanish (Latin America): Ustedes están en la escuela.

Están comes from the verb estar, which we use to talk about physical location. Whenever you are stating where someone or something is located, you must use estar, not ser.

Does ustedes mean a formal 'you' or a plural 'you'?

In Latin America, ustedes is the standard plural 'you'. It is used whenever you are talking directly to more than one person, whether you are speaking to close friends (informal) or to strangers (formal).

Why does ustedes use están, which is the same word used for 'they'?

In Spanish, the pronoun ustedes always shares the exact same verb forms as ellos and ellas (they). Because of this rule, están can mean 'they are' or 'you all are' depending on the pronoun used before it.

The English translation says 'at the school'. Doesn't en mean 'in'?

The Spanish word en is very flexible and can translate to 'in', 'on', or 'at' depending on the context. Here, en la escuela simply tells us that you all are located at or inside the school.