Learn Croatian: Došao si u pravi čas, ručak je gotov. - You came at just the right moment, lunch is ready.

The preposition u takes the accusative case when it indicates a specific point in time or a destination. It only takes the locative case for a static location, like being physically inside a place. Because čas is a masculine inanimate noun, its accusative form looks exactly the same as the nominative.

Does the word čas mean a school class here?

No, in Croatian, a school lesson or class is almost always called a sat. The word čas usually means a moment or instant. The phrase u pravi čas is a very common set expression meaning 'at just the right moment' or 'in the nick of time.'

How do we know the person being spoken to in this sentence is male?

We know this from the verb došao. In Croatian, the past tense participle changes based on the gender of the subject. If you were saying this to a female, you would need to use the feminine form and say došla si.

Why is the adjective gotov used instead of gotovi?

Gotov is the indefinite form of the adjective. In Croatian, when an adjective comes after the verb 'to be' to describe the subject, you generally use this short, indefinite form. You would use the definite form gotovi if it came directly before the noun to mean 'the finished lunch.'