OK listen
My suggestion is:
Don’t change the surname. Don’t hide your identity.
If you change your surname, that means you have surrendered in front of casteist big0ts.
Say that you are a proud dalit.
They aren’t your friends.
Suppose you will change your surname, and then they won’t tease you. But they will bully some other dalit guy.
Confront them. Be bold. Set an example. You can save many dalits from being mocked by them in the future.