Learn Italian: Tutti gli studenti si allenano in piscina. - All the students work out in the pool.

When you use tutti to mean "all", you must include the definite article (like gli here) right after it. You literally say "All the students". This is different from ogni (every), which connects directly to a noun without an article.

Could I use ogni to say "Every student works out" instead?

Yes! But remember that ogni is always followed by a singular noun and verb. You would say Ogni studente si allena in piscina. The meaning is practically the same, but the grammar switches from plural to singular.

Why is there a si before allenano?

The verb here is allenarsi (to train / to work out), which is a reflexive verb. The si is the reflexive pronoun for "they" (loro). It literally translates to the students training "themselves".

Why is it in piscina and not nella piscina?

For many common locations in Italian, such as a pool (piscina), a bank (banca), or a church (chiesa), you just use the preposition in without combining it with "the". You would only use nella (in + la) if you were talking about a very specific pool, like "in the new pool".