Don't say:
A girl has a pony who is in our class.

Say:
A girl who is in our class has a pony.

Note:
Enclose a relative clause that may be omitted between commas. My brother George, who is in another class, has a new bicycle, A relative da use that can't be omitted is not enclosed within commas: The boy who spoke to me is my brother. Put the relative clause immediately after the noun to which it refers.