Learn Italian: Ci sono quattordici studenti e un insegnante. - There are fourteen students and one teacher.

In Italian, the numbers 11 through 16 are formed by putting the base number first and adding -dici (meaning "ten") at the end. So quattordici is quattro (four) + -dici. Be careful, though: for 17 to 19, the order flips and dici- comes first, like in diciassette!

Why is it Ci sono instead of C'è?

Ci sono means "there are" and is used because we are talking about a plural group (quattordici studenti). If there was only one student, you would use the singular form c'è ("there is").

Does un mean "one" or "a" in this sentence?

It can be both! In Italian, the word for the number "one" and the word for "a/an" are exactly the same (un, uno, una, un'). In this sentence, it translates perfectly as either "one teacher" or "a teacher".

How can you tell if the insegnante (teacher) is a man or a woman here?

You can tell by looking at the word un! The noun insegnante is the same for men and women. If the teacher were female, it would be written with an apostrophe: un'insegnante (because the feminine una drops its "a" before a vowel). Since there is no apostrophe here, we know the teacher is male.