Learn Italian: Una buon'alunna aiutava l'insegnante. - A good pupil helped the teacher.

Because alunna is a feminine noun starting with a vowel. The feminine adjective buona drops its final -a and adds an apostrophe to make pronunciation smoother. Keep in mind that for masculine nouns starting with a vowel, you just use buon without an apostrophe (for example, un buon amico).

I learned that feminine words starting with a vowel use un'. Why does this sentence start with una?

The indefinite article always matches the word immediately following it, not necessarily the noun itself. Since the adjective buon' starts with a consonant (b), we use the standard feminine article una. If the noun came immediately after the article, it would indeed be un'alunna.

Why does the sentence use aiutava instead of ha aiutato?

Aiutava is in the imperfect tense, which is used to describe ongoing, repeated, or background actions in the past. It translates best as "was helping" or "used to help" the teacher. If it used the passato prossimo (ha aiutato), it would mean she helped the teacher once during a single, completed event.

Is insegnante masculine or feminine in this sentence?