Elle est étudiante.

This verb is irregular — it isn't built from anything and has to be learned. Its full table is below, in the rule's explanation.

Mood

indicative

Tense

present

Person

third person singular (il/elle/on)

With

Elle

Irregular. There is no stem-plus-ending pattern to follow; the whole table has to be learned, like English be: am, is, are.