Learn French: Vous êtes rapides, mais les étudiants sont lents.

Yes! Normally, the s at the end of les is silent. However, because étudiants starts with a vowel, you must pronounce the s as a z sound to connect the two words. This smooth connection is called a liaison.

Does the article les change if the group of students is entirely female?

No, it doesn't! Unlike the singular articles le (masculine) and la (feminine), the plural definite article is always les, no matter the gender of the noun. If it were an all-female group, the noun itself would change to les étudiantes, but the article stays the same.

Why is there an s added to the end of rapides and lents?

In French, adjectives have to match the noun they describe. Since vous (you all) and les étudiants (the students) are both plural, the adjectives describing them must also be plural. We add an s to rapide to make it rapides, and to lent to make it lents.

Since vous can also be used to address a single person respectfully, how do we know it means 'you all' here?

You can tell by looking at the adjective! While vous is often used formally for one person, the adjective rapides has a plural s at the end. If the speaker were talking to just one person formally, it would be written as Vous êtes rapide without the s.