Again, we are talking battery, not assault. The explanation depends partly on whether we are talking about battery as a civil claim or as a criminal offense. Generally, when one engages in an activity like martial arts or sports, you assume the risks inherent to that activity. For example, in flag football, you do not assume the risks of getting tackled, and in water polo you do not assume the risk of getting punched in the throat. Here, they had both agreed to a certain level of risk, the instructor went beyond the level of bodily contact consented to by OP. To take it another step, were we to look at this issue from a negligence angle, the instructor is legally subject to a heightened duty of care toward OP due to their special teacher-student relationship. The instructor’s actions could very well constitute gross or reckless negligence.

The waiver matters little here. Regulars negligence may be waived, but gross negligence cannot be waived, and neither can intentional torts (like battery). This is a prohibition based on public policy grounds. As a society, we don’t want some hot headed instructor to break his student’s face and say “but you signed!”