Learn Portuguese (Portugal): Ela é a professora e não é alta.

In Portuguese, to make a statement negative, you simply place the word não directly in front of the verb. So, 'is not' translates to não é (literally 'not is').

What is the difference between é and e in this sentence?

é (with an accent) is a verb meaning 'is', while e (without an accent) is a connecting word meaning 'and'. The accent also changes the pronunciation: é has an open sound like the 'e' in the English word 'pet', but the connecting word e sounds more like a short 'ee' or 'ih'.

Do I need to say ela again before não é alta?

No, you don't. In Portuguese, it is very common to drop the subject pronoun (like eu, tu, or ela) once it is clear who you are talking about from the context or the first part of the sentence.

Why is the word for tall written as alta instead of alto?

Because describing words (adjectives) must match the gender of the person they describe. Since the subject is feminine (ela), the adjective takes the feminine ending -a to become alta. If the subject were a man (ele), it would be alto.