From description of Red Wizard Feat:
Taking this feat will prevent you from casting any more spells from this spell school. This is required by the Red Wizard, as you must lose an extra school upon entering this class. One school should be the opposite of your specialist school, the other is players choice.
Well... this is how I understand it:
1. you start as a specialist wizard (1 oposing school, sometimes 2 if PnP schools are enabled)
2. you advance as a red wizard of thay - to do that, you give up an extra spell school - now, with PnP schools off, you select the school that is oposing to your specialist wizard (loose nothing in here) and one extra school - so you give up 2 spell schools.
With PnP schools instead of 1 spell school you have to select 2 schools which must be different from your specialist wizard oposing school(s) - so you give up 3 - 4 spell schools!
My point is that with PnP spell schools enabled you have to give up one more spell school than normally.