I would say ice if you want to do pvp at a high level but you don't want to wait until max level. Storm if you want to wait until max level (although you don't have to be max level, shadow spells will help). I think you should try doing pvp with your myth because you can buy medusa treasure cards and use those (if you don't already have the spell). Lastly, don't delete your myth, you might use her sometime (unless you have five other wizards you don't want to delete. Saffron Sandsword Legendary
I have decided to delete my myth wizard and start a new wizard, only for PVP. What school should I do?
As cliché as it may sound, the best school for PvP is the one that you enjoy the most. If you don't like your school, you won't be as determined as you would be with one you prefer. That said, your question should be more specific. What level do you want to PvP? Do you want to do 1v1 or group matches?