I wonder wich level this is? Highschool? Uni? If you know the teachers you will know if they expect some kind of controversy in the essay or if you should keep it nice and good. In the latter case I would suggest fairytales or classical beeings like the easterbunny or as mentioned before: Santa. Not much controversy there and you should be able to wrack your mind into a 350 words essay in no time at all. If you think you will score extra points/get better chance of joining the class with a "not so PC subject" I suggest something like "Crime does'nt pay" (wich often shows to be wrong). This is usually a subject one should not touch with a 10ft pole if one does'nt know that the teachers are open to a "red hot" subject like this.
Good luck