My Condolences. Personally I'd go. School is important, but most of it you can do over if absolutely necessary. I would also think most professors would be understanding, and pardon you. Your grandfather however, will only have one funeral, and that's it. And personally, I think family in situations like this is more important.
My $.02.
Also, who's scheduling the funeral? When my grandma died, my parents scheduled it to be on a Saturday so I wouldn't have to miss school. Don't know if that's a possibility for your family.