I expect most GMs mandate that the elf wizard favored class bonus adds extra uses to an arcane school special ability that the elf wizard already possesses. This 2012 thread on the same topic cites Pathfinder designer Jason Bulhman's post that says that, essentially, You can't add something to nothing. That is, adding +½ to the number of uses of an ability a creature doesn't possess doesn't grant the creature the ability!
However, a strict legalist could argue that the elf wizard favored class bonus specifically says Select one arcane school power at 1st level that is normally usable a number of times per day equal to 3 + the wizard's Intelligence modifier, and that sentence does not limit the elf wizard only to special abilities he's already selected, therefore leaving open the option to select via the elf wizard favored class bonus a new, previously unpossessed arcane school special ability.
For example, an elf wizard that specializes in abjuration at level 1 gains the supernatural ability protective ward usable a number of times per day equal to 3 + his Intelligence modifier. At level 1, using the elf wizard favored class bonus, the same elf could select the universalist wizard supernatural ability hand of the apprentice. However, it would be up to the GM if the elf wizard were, at level 1, able to use the ability hand of the apprentice either a number of times per day equal to 3 + his Intelligence modifier or 0 times per day (i.e. ½ rounded down), this latter reading stemming from the GM reading the elf wizard favored class bonus number of uses specifically overriding the typical number of uses for the special ability.
Despite this second reading being possible in the very abstract, I wouldn't expect any GM to agree with it without substantial persuasion in the form of booze, cash, or pizza.