Until the 1990s strength and power training was typically not recommended for youth (children or adolescents) because of fear of injury. Research conducted in recent years indicates that most injuries occur as a result of inappropriate use of equipment and lack of qualified supervision. In 2009 the National Strength and Conditioning Association published a position paper on youth resistance training. The reference to this position paper is at the end of this answer.
Plyometrics is one of many types of resistance training. It involves the use of an eccentric muscle action in which the muscle lengthens followed by a concentric muscle action in which the muscle shortens. Think about a basketball player who attempts to block a shot during a game. The player bends her knees and hips as she lowers her body (eccentric phase) and then jumps up as quickly as possible to block the shot (concentric phase). Children do plyometrics when they jump rope or play hopscotch.
Plyometrics is not limited to jumping; it can be done with the arms as well as the legs and is most effective for improving power (strength × speed). Done properly, resistance training, including plyometrics, can be safe and beneficial for children and adolescents (see reference).
So is jumping over small boxes (about 12 inches high) appropriate for middle school youth who have been in physical education for several weeks? The position paper on youth resistance training suggests “that you begin with a light load and progress the training program depending on needs, goals, and abilities. Listening to the needs and concerns of individuals is essential” (page S62).
Avery Faigenbaum, senior author of the position paper on youth resistance training, indicates, “While plyometric training that is sensibly progressed over time has been found to enhance movement biomechanics and improve functional abilities in school-age youth, injuries can occur if the intensity, volume, or frequency of training exceed the abilities of the child. The key is proper instruction on both jumping and landing mechanics, which should be part of PE.”
So what is appropriate for one youth might not be for another. If power is a goal, plyometric exercise such as jumping over 12-inch obstacles (preferably not boxes) could be appropriate for some young athletes who have participated in a progressive plyometric training program, but this type of exercise will likely be too intense for most middle school students in PE. It depends on individual abilities, training experience, and the quality of the movement. The use of elastic cords set at varying heights or different-sized cones would be safer, and the height could be adapted to the needs of the individual.
Reference: Faigenbaum, A.D., et al. 2009. Youth resistance training: Updated position statement paper from the National Strength and Conditioning Association. Journal of Strength and Conditioning Research 23(supplement): S60-S79.