Learn Italian: Luca è un buono scolaro.

The form of buono changes depending on the letters of the noun that follows it. Because scolaro starts with an "s" followed by another consonant ("c"), buono keeps its full ending. If the noun started with a vowel or most single consonants, it would drop the "o" and become buon (like buon amico or buon libro).

Wait, doesn't a word starting with "sc" require the article uno? Why is it un here?

Normally, yes! You would say uno scolaro. However, the article always matches the word immediately next to it. Because the adjective buono steps between the article and the noun, the article must match buono. Since buono starts with a simple consonant ("b"), the article becomes un, giving us un buono scolaro.

Could I put the adjective after the noun and say uno scolaro buono?

You could, but it changes the meaning slightly. When buono comes before the noun, it means "good" in a general sense (a capable, well-behaved pupil). Placed after the noun, buono tends to emphasize being a "good-hearted" or "kind" person.

What is the difference between scolaro and studente?