Are those who follow the Sunni schools Maliki, Hanbali, Shafii and Hanafi) or the Shia schools (Zaydi, Ibadhya and Jaafari) Muslims? What is the ruling on takfir (declaring someone an unbeliever or kafir who is or claims to be Muslim.) [See glossary for an explication of terms.]
1. Yes, anyone who follows one of the schools mentioned before in the question is Muslim. They have the same rights and duties that the Muslims have.
2. It is not permissible to declare anyone who follows one of these schools an apostate or those who follow the Ashari belief (see glossary) or the real Sufism (see glossary) or anyone who believes that there is no God but God and Muhammad is his prophet and recognize the pillars of Islam.
[Note: For similar opinions, search: Amman Message.]