If the school has an official name it is currently using while trying to decide a new, permanent name, then you would say the currently-used name is "temporary."

If the school doesn't have an official name yet but does have an idea for a name that the school board likes and might decide to use permanently (for example, they might vote on a permanent name next month at the board meeting), then the appropriate label is "tentative." 

There is also a "working name," which would be a name that is official but with the idea that the eventual, permanent name will most likely be different. It's very similar to "temporary name."