... Lifespan and Lifeforce are different things? Lifespan denotes when you die.. So if you run out, something happens to kill you, not that you die due to your body failing, but the universe literally will kill you in the most strange ways possible. No Life Span means you are effectively going to die. The only question is how your life is going to end.
Lifeforce Drain is literally draining the Life out of someone, meanwhile.
[Insert Irony's avatar here]
On the brute force side of things, you're only casting at +2, which is generally insufficient to actually murder someone (important and skilled) in one turn. Mooks yes, but major characters- such as your fellow students- are generally wriggling enough that you can't expect to bring your full force to bear on them so easily. The downside here is that they can be obnoxiously difficult to kill. The upside is that you can be obnoxiously difficult to kill.
On the underlying principle side of things, Precepts generally can't really "invent stats" like that- eg you can't use Death to boost somebody's performance by killing their incompetence, or Life to wither them to a husk by growing their mortality. For similar reasons, you also can't use Life to nurture their careers or, as in this case, Death to reduce their preordained day of reckoning. There's certainly ways to inflict curses on people, but they require building their intended effects directly, rather than inventing and then leveraging a more convenient concept.