Endless money and time would enable EVERYTHING I'd make a difference in, so honing it down to just one for the sake of space would imply that one is my highest priority, which it's not. They're all high priority.
One I'd do is establish a prep school for girls from daycare through eighteen. It would omit the common attraction to and distraction of boys, encouraging serious study and attaining serious attention. There would be jumpers and Tshirts as dress code, to omit competition with wardrobe, and closed-toed shoes for toe protection. Classes would start at 9 and end at 5, beginning at age 15. There would be Latin, French, math, science, English, civics, geography, history, art and music, all in a Montessouri-type approach allowing free rein to high achievers and patient repetition for low ones. Rather than grades, there would be proficiency levels. Age, but for hours of 9 to 5 at 15, would be no criteria for placement in proficiency levels. Specialties would be offered for the last three hours after age 15, in whatever field proficiency warranted. Accredited post-graduate study establishments, such as colleges and universities, issuing their standard SATs and such, would ideally also accept proficiency levels rather than grades.
I'd do the same for boys, with dress code pants, belt and shirt.
There would be sponsored socials so the sexes could mix. Pregnancy and any disease would be avoided with birth control for both sexes and a thorough education about various means of contracting those diseases.
The result I would hope, would be thoroughly educated young men and women who realize first-hand how many years are available for further improvement before partners for families or permanent companionship are chosen.