Learn Portuguese (Portugal): Os alunos estão na escola.

In Portuguese, there are two verbs for "to be": ser and estar. We use estar for temporary states and to say where someone or something is located. Since this sentence is about the physical location of the students, we use estão (the "they" form of estar).

Where does the word na come from?

It is a combination of two words. When the preposition em (which means in, at, or on) is followed by the feminine word for "the" (a), they merge together to become na. This happens because saying "em a escola" is clunky to pronounce.

Why does it literally translate to "in the school" (na escola)? In English we just say "at school".

Portuguese uses definite articles (words for "the") much more often than English. When talking about common places you go to, like a school, supermarket, or bank, you almost always include "the" (a or o) before the noun.

If there are female students in the group, is it still Os alunos?

Yes! If a group of students is mixed (both male and female), Portuguese defaults to the masculine plural form (os alunos). You only use the feminine plural (as alunas) if every single person in the group is female.