What’s a good Power/Strength Training for a 12-14 year old?

This depends on the level of physical preparedness of the youth.

GPP must be established prior to engaging in power and strength development.

The general progression would go from calisthenics/calisthenics with added resistance then to training with weights. Plyometrics may be integrated at an early age do to the plyometric nature of children at play.

Med ball throws, jumps, OLs (technique).

Children adapt more favorably to strength training post puberty (It is therefore important to make the distinction between actual and biological age)