If your son suffers any injury due to improper training or supervision, the teacher, the school, and the district could be liable. However, that is not the preferred outcome--you don't want your son to suffer injury but then have the possibility of compensation. You should probably get a doctor's note that your son is experiencing knee pain due to the training; if this teacher will not take that seriously, escalate to the principal and even district level if need be, letting them know that if they cause your son injury, you will sue. This may make them re-evaluate the class.