Should I include high school details in Grad School Resume?

I have to disagree slightly with the stark "No" answers.

Only when those details provide evidence of your potential for research.

Some information about your high-school experience may shed light on your research potential. Certainly you would want to mention winning the Intel Science Talent Search or the International Olympiad in Informatics, or describe the peer-reviewed research papers you published as a high school student.

But your high school grades? No, leave them off.

No

You already explain why: they only ask for undergraduate details. The graduate school admission committees already have too many applications to look at. Don’t over-load them

Your idea is actually counter-productive. If your high school record is better than your undergrad credentials, they would wonder why your academic achievement is regressing instead of progressing.

No.

It's actually a bad idea to include documents which are not actually required. Some universities even specifically mention not to send documents which are not required.

And also as @scaahu explained, it will give an negative impact as you did poorly in undergraduate compared to high school.