Do mezzo sopranos have their own sections in church choir or are they usually kept in the alto sections?

My cousin and I are both in church choir, but her voice frustrates her. She said she's in between being an alto and soprano, or a mezzo-soprano for short. She can sing alto fine, but she starts singing soprano notes sometimes. When she sings soprano, she can't hit the high notes. Are there such things mezzo-sopranos in church choir? (PS- NO WEBSITES, JUST ANSWERS!!!!)