Technically you can't take it at all, since it requires you choose a familiar as your arcane bond at first level, which you can't do since you don't have that feature.
I personally would be willing to overlook that, since you will be getting a familiar at third.
Nothing in VMC letting you choose a school get around restrictions of the school though, so you would indeed have to be of the correct enlightenment.
Planar Knowledge (+3 to Knowledge: Planes) is one of the first level powers of the school, so you would get it at 7th level.
You if you already had improved familiar (hopefully having chosen an imp to not make thing difficult on anyone) then you would get an ability at 19th level that doesn't do anything, but you would have a feat you could retrain for something else without loosing your Imp.
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As the GM, I would beat you :)
Also, I would question why you want this school and why you are trying to make things hard.
As far as the rules go, there isn't a particular answer as to what would happen to the familiar you had previously, but when you reach 19th level you must have an imp familiar and the other one therefore has to go. If I personally had to decide what happened, I would probably have the new Imp kill your old familiar.