I tried some boxing while in the best shape of my life in HS and it drained me incredibly fast. Comparable to any sustained aerobic activity IMO, complicated by the fact someone is literally trying to hit you in the face. Mentally and physically draining.

We had a gym unit on it and our teacher let anybody fight who brought in a signed permission slip. They did not appear drafted by the district's legal counsel. It was an awesome unit, people would skip class ro come watch the matches. I imagine few schools offer it these days.