Any eating disorder can cause death. You don't have to be emaciated for your heart to give out. Organ failure can happen as a result of electrolyte imbalances. It is not so much the actual bmi that determines the state of your organ health, more the actions that you put yourself through day after day after day. That is what will potentially kill you.
I once dropped to a bmi of 11.9 and nearly died as a result. Yet I also nearly killed myself at a higher bmi from an overdose. Remember this important fact, many eating disordered people who die, actually die by their own hand. i.e. they commit suicide because the weight of the pain they carry around in their hearts becomes too heavy a load.
So, there is not a cut off for when your weight will kill you. If you have an eating disorder of any type, then you are literally dancing with death.
Good Luck with your school work.