[See glossary for terms.]
1] Anyone who follows one of the Suni and Shii school is a Muslim and no one can declare him/her an apostate. According to a fatwa that was issued from the sheikh of al-Azhar (a leading Islamic institution in Egypt), no one can declare anyone who follows the Ashari School or the right Sufism. Also, we cannot declare an apostate anyone who follows the real salafi thought (see glossary). So it is not permissible to declare anybody or any category who believes in God and that Muhammad is his prophet and in the pillars of faith and respects all the pillars of Islam, an apostate.
2/ All the eight schools, either the Sunni or the Shia ones, share the same fundamental principles of Islam. They all believe in one God, Muhammad is his prophet. Also, they all agree about the 5 pillars of Islam and Pillars of faith which are: believing in God, in his angels, in his books, his prophets, the day of judgment, and the destiny. The differences are in some branches of jurisprudence and not in the roots, which is a good thing.
3/ It is not permissible for anybody who does not have the knowledge to deny a fatwa or give a fatwa without adhering to the methodology of the schools. Also it is not permissible of anybody to claim ijtihad (independent reasoning) and create a new school or give a fatwa which can lead Muslims to things that do not match the sharia rules.