The research I've seen makes it easy to draw some important conclusions about the learner's permit:
1. Crash rates during the time teenagers are learning to drive, with their parent in the front seat with them, are extremely low.
2. Although the role of the YOUTH of the new driver in the increased crash rates in new drivers is debatable, there's no question that INEXPERIENCE is a major factor, and probably THE major factor. I'd add to this the related observation that we do a terrible job in the USA providing adequate driver training for new drivers. They need training and they need a lot of experience.
This is why I think it's a mistake for parents to hold the learner's permit ransom to things like academic performance and "being more responsible." In my opinion, which is not shared by all, parents ought to encourage their kids to get that permit as soon as possible and then start thinking of the task of teaching their kids to drive as a part time job.
Teenagers holding learner's permits often complain that they have to ASK or even beg their parents to give them driving practice time. Why? Why would parents have to be asked? Why would parents short-change their own children in teaching them this critical life skill?
Get the permit as early as possible. Require that it be held for one full year before allowing your teenager to get a drivers license. During that year, provide lots of training time and give your new driver what he probably needs most to be a safe driver: Experience, experience, experience.