I would recommend writing out all the possible entries you could include on your application first. That way, you can see everything in front of you (perhaps just by titles or a few bullet points, ultimately wanting to get the idea of each entry). Then try to categorize activities into leadership, service ethic, etc. Seeing all your activities at once will also help you prioritize things. If it’s the case that you still have space after including all post-secondary activities, then definitely include high school activities too.
Having said that, you may have a stellar high school activity that is way better than certain post-secondary ones — if that’s the case, then definitely prioritize your high school one.
At the end of the day, like @JohnGrisham has said, there is nothing wrong with including high school activities. So, go for it .