First thing you should understand about SC and ST is their full forms.
SC- Schedule Caste
ST- Schedule Tribe.
Buddhism is a religion just like any other religion of the world, like the Hindu, Muslim, Sikh , Christian , Jain, Ayyavazhi etc.
The reserved castes or tribes are a small part of a religion.
Lots of castes makes one religion and the castes which are of lower standard.
According to Google Meaning of Schedule Caste- the official name given in India to the lowest caste, considered 'untouchable' in orthodox Hindu scriptures and practice, officially regarded as socially disadvantaged.
So do not get confused for Buddhism to be SC or ST. It's neither a caste nor a tribe but a big religion.
Buddhism have 376 millions of followers worldwide.