Hello,

Yes, your SC category certificate is absolutely acceptable because as you mentioned in your query it is issued by tehsil authority. So I am assuming that this tehsil is of the state Uttar Pradesh then that would not be a problem and the reason is that Varanasi and Tehsil both are cities of the same state. Incase you apply out of your home state then you need to get a category certificate issued by the central government.

And the validity is up to 3 years and after that you would have to renew it again.

Hope it helps.

Good luck.