You know for sure that the curse didn't get you, but you don't know which students can't be trusted, and because the possessed students may lie only occasionally, there is no guaranteed way to test them to determine which are cursed.
Sabes con certeza que tú no tienes la maldición, pero no sabes en qué estudiantes no se puede confiar, y como los estudiantes pueden estar poseídos solo ocasionalmente, no hay una forma garantizada de ponerlos a prueba para determinar quiénes son.