You can also use the enhanced specialization (renamed Arcane Focus in Towers of High Sorcery) to become a specialist at that point. Or you can just not bother, and remain a generalist.
Or, you can not bother to take the Wizard of High Sorocery prestige class at all - it's not required. As long as your character takes the Test and doesn't seriously cheese off the leadership of the Orders, he's a member whether he takes levels of the PrC or not.