In order for something to be a symbol, it must be have both literal meaning and figurative meaning. Therefore, there would actually have to be white elephants present in some literal way, and then they'd have to carry some figurative meaning on top of that (such as referring to something that is generally unwanted, or a burden, as other educators have said), in order to function as a symbol. Since there is no literal white elephant, it is not a symbol.
That being said, a simile is, technically speaking, literal, because a simile only says that something is like or as something else, not that it is something else (as a metaphor does). If the title of the story were "The Hills Are White Elephants," this would be a clear-cut metaphor). At the same time, however, I have sometimes seen the simile, as a figure of speech, lumped in under the broader category of "metaphor" because similes are similar to, though less powerful than, strict metaphors. Therefore, if you are only given two options—either symbolic or metaphorical—I would place the title in the metaphorical category.