Learn French: Les étudiants sont libres maintenant.

In French, adjectives must match the noun they describe. Because les étudiants (the students) is plural, we add an s to the base adjective libre (free) to make it plural as well.

Do I pronounce the s at the end of libres?

No, the s at the end of libres is completely silent. You pronounce it exactly the same as the singular form, libre. The s is only there in writing to show that the word is plural.

Is libres in the masculine or feminine form here?

It works for both! The base adjective libre already ends in an e. When a masculine adjective already ends in an e, it stays exactly the same for the feminine form. We just add the s to make it plural for any group.

Why does it sound like there is a 'z' sound before étudiants when spoken?

This is because of a pronunciation rule called liaison. Since Les ends in an s and étudiants starts with a vowel, the normally silent s on Les wakes up and is pronounced as a 'z' sound to smoothly link the two words together.