Learn Spanish (Spain): Hay cuatrocientos estudiantes en la escuela. - There are four hundred students at the school.

In Spanish, the word hay means both "there is" and "there are". Unlike English, it stays exactly the same whether you are talking about one thing or many things.

Why does the number cuatrocientos end in -os?

Numbers in the hundreds (from 200 to 999) must match the gender of the noun they are counting. Since a general or mixed group of estudiantes is treated as masculine in Spanish, we use the masculine plural ending -os. If you were talking about a group of only female students, it would be cuatrocientas.

Is the word estudiantes masculine or feminine?

The word estudiante doesn't change its spelling based on gender. A male student is el estudiante and a female student is la estudiante. For plural groups, Spanish always defaults to masculine unless every single person in the group is female, so we treat it as masculine here.

The translation says "at the school", but isn't en the word for "in"?

Yes, en translates to "in" or "on", but it is also the most common way to say "at" a location in Spanish. Because Spanish doesn't have a separate preposition just for "at", en la escuela works perfectly for "at the school".