Nope. In this case it's +1 Power Level of cast spells i.e. the spells you can (from that school) will be slightly more powerful. It doesn't mean you have access to spells of a Power Level higher (yeah, Obsidian used Power Level to mean two different, but related, things).
Thanks, this is the kind of thing that can make a significant impact on class choices at the start of the game.
In retrospect, they could have just differentiated between 'skill level' and 'power level' to make it easier to understand.
I could imagine there being a place for a wizard multi-class that specializes in a school and hence looses two schools for the benefit of preserving most of one, but based on this, the wizard sub-classes look quite unappealing now.