Best school? Best guitar? Practice every day? All helpful. But not if your kid doesn’t want to play. Kids who do well are in families that place a value on playing music:

The instrument is easy to get to and in good condition
No one is nagging them to practice
No one is saying “Stop playing the same thing over and over!”
Family members (parents, mainly) listen in a general way, from another room, while kids play; comments and questions afterwards.

Someone is interested in what the child is playing, and verbally noting their progress.