The main difference between estar and ser is when they are used - estar is used for states which aren't permanent, meaning they can change. For example, a famous sentence from Mexican telenovelas: Estoy embarazada, which means - I am pregnant. It is not a state that will last forever. Having that in mind, here are the correct answers:
1. Nosotros estamos en la piscina.= We are at the pool.
2. Tu estás en la escuela. = You are at school.
3. Mi hermano y yo estamos en la cocina ahora. = My brother and I are in the kitchen at the moment.
4. Yo estoy en el comedor ahora. = I am in the dining room at the moment.
5. Hoy es Martes. = It's Tuesday today.
6. Son las tres de la tar.de. = It's three in the afternoon. (for hours, the verb ser is used).
7. Que hora es? = What time is it?
8. Estas tu contento hoy? = Are you happy today?
9. Hoy es Domingo. = It's Sunday today.
10. Los amigos son italianos. = My friends are Italian.
