she's turned on by the prospect of her life becoming more exciting after this, but also she wants at least one student to remember her, forever
she's turned on by her new born savior complex
saving this boy makes her feel renewed, sexy, wanted, special, important, ... you name it
at first she may have just interfered out of a sense of duty as a teacher, but now she holds power over him - she now has monopoly over this boy's suffering, and he's dependent on her
pretty interesting development