Start with enough money so you can build without asking anyone's say-so.

Recruit both scholars and natural talents in various fields, from mythology to comparative religion to Petecostal tongue-speakers to parapsychologists and crypto-zoologists. Find druids, witches, fakirs, herbalists, reiki practitioners and faith healers.

Build a comprehensive curriculum, emphasize practice over rote memorization, but make it a really scholarly curriculum. Include lots of tests and research. Have the curriculum include arts and philosophy, energy theory and aspects of bilogy. Eventually, students will appear who are both natural talents and good students. They will achieve results. Make them teachers for the next round. Keep expanding the boundaries of what "magic" is and what should be taught and where teachers and students should be recruited.

Add a few hundred years. Refine the curriculum as a better idea of what works and what doesn't develops. Do not judge or mandate anyone's spiritual beliefs, just teach them the stuff.

And there you have it. How to build a magic school.