"Since Zaidiyyah shares similar doctrines and jurisprudential opinions with Sunni Islamic scholars, Zaydis are even described by some analysts as the fifth School of Sunni Islam"
Zaidiyyah
See pg 214:
Sectarian Politics in the Persian Gulf
See pg 23:
Yemen
Pg456:
Historical Dictionary of Yemen
Even described as isn't really saying they ARE.
They reject the righteousness of the first three Caliphs and uphold the Imamah.