Learn Arabic (Modern Standard): هناك سبعة تلاميذ في المدرسة.

This is the reverse agreement rule for Arabic numbers 3 to 10. The singular word for pupil, تلميذ, is masculine. Because the noun is masculine, the number takes the feminine form سبعة instead of the masculine form سبع.

Why do we use the plural تلاميذ instead of the singular?

In Arabic, when you count from 3 to 10, the noun being counted must always be in its plural form. Here, we use the broken plural تلاميذ instead of the singular تلميذ. In grammar terms, this plural noun is put into the genitive case, which means the phrase literally functions a bit like saying 'a seven of pupils'.

If I want to talk about seven female pupils, how does the number change?

The singular for a female pupil is تلميذة, which is feminine. Because of the reverse agreement rule, the number would drop its feminine ending and become masculine. You would say سبع تلميذات.

Do I need to change هناك since we are talking about a plural subject?

No, هناك is incredibly versatile! It is used for both 'there is' and 'there are'. It stays exactly the same whether you are pointing out one pupil or seven pupils.