Learn French: Les écoles sont jolies. - The schools are pretty.

Yes, you pronounce the 's' in les like a 'z' sound here. Because écoles starts with a vowel, the normally silent 's' in les connects to it. This connection is called a liaison, and it makes the phrase sound like 'lay zay-kohl'.

Could I just say Écoles sont jolies without Les to mean 'Schools are pretty'?

No, you cannot drop the les. In English, we often drop 'the' when talking about things in general, but in French, nouns almost always need an article (like les) in front of them.

Why does jolies have an 'e' and an 's' at the end?

Adjectives in French must match the word they describe. The word école (school) is feminine, so we use the feminine form jolie. Since there is more than one school here, we add an 's' to make it plural, giving us jolies.

Why do we use sont here?

sont means 'are'. It is the 'they' form of the verb être (to be). Because 'the schools' is a plural group, French treats it the same way as 'they' (elles), so we must use sont.