The short answer is that it's not specifically explained in the show, however we can make certain assumptions about his actions.
Principal Wood deliberately took the job because he knew that "something big" is coming. He's been actively fighting vamps and demons for decades and can smell which way the wind is blowing.
PRINCIPAL WOOD: Well, I actually do enjoy the work, but yeah. Yeah, you're right. I maneuvered myself into that school, that
office — just like I maneuvered you there. The hell mouth draws the bad
things in close, and now we're headed for something big, Buffy. Really
big, and I need to be here when it happens. I want to help.
Buffy: First Date
Wood knows that the seal is important. He takes great pains to rebury it before taking Jonathan's body a considerable distance away from where he found it. Calling the police would undoubtedly result in the area being investigated, dug up and possibly the seal being opened by a curious (but clueless) constabulary. None of which are things that he wants.