There is no rule that you can't include HS activities, but they won't be much regarded. Including them might be looked on as "fluff." Or not, depending on the individual adcomm.
Alternatives: If you have any other job or activity related to construction during the college years, you might mention the HS jobs in the same space to give context, if you like (like Habitat for Humanity, for example) rather than giving them their own space. Alternatively, you could mention those long-ago jobs somewhere in your PS if you can relate them to your pursuit of medicine.
About nine activities is the average listed, including hobbies and artistic endeavors. Less than five might look sparse, but I'd suggest looking for other ways to fill spots if at all possible.