Yes grandparents meet the legal definition of immediate family

I would ask and if they say no then just call your kid in sick that day. Or just call in sick and don't ask.

Even if the policy doesn't include grandparents as "immediate family" I can't imagine a scenario in which a school would not consider a funeral for a grandparent to be an excused absence.

Is attendance at a grandparent's funeral an excused absence? The policy says "immediate family" but there is no definition. I'd hope that attendance secretaries would be sympathetic.