Jenny was killed in the episode, "Passions" on February 24, 1998. She was killed by Angel, while trying to 'cure' him. And give him back his soul. A thing that Jenny had tried to do for so long. And she never got to finish.
Angel killed Jenny by 'snapping her in half', Angel broke her back. Before she died, Jenny had finally reconciled with Giles, and was planning on going to his house, once she finished her spell. She finally had the incantation translated, when Angel got in.
"The sign outside the school..." "Enter al ye who seek knowledge..." "Yeah, what can I say? I'm a knowledge seeker."
Jenny had just saved the information on a disk, when Angel smashed the computer, and the printout. He also smashed the glass ball, the Orb of Thesila, against the blackboard. Eliminating all chances of his soul being returned - except one.
He overlooked the disk. The disk is currently on the floor between Miss Calendar's desks. If someone finds it they could restore Angel's soul. But no one knows where it is...
When Giles came home he expected to find (a live) Jenny, especially since there was flowers and wine set out at his house. Also, a note saying, "Upstairs". But, this was all orcastrated by Angel. When Giles went upstair he found a dead Jenny on his bed. I think it was one of the saddest moments. Angel also left a drawing of a dead Jenny. It was even sadder (and brought tears) when Giles called Buffy and Willow with the sad, sad news. They were crying and shocked, it was soo sad.
Giles went to avenge Jenny's death and attacked Angel in his home. When Buffy learned of his plan, she bailed him out. But Angel got away. Xander yelled at Buffy because she never killed Angel, but the girls were simpathetic.
Giles and Buffy were safe, but the entire gang is in great mourning. Buffy and Giles put flowers on Jenny's grave and Buffy apologized, for not killing Angel. Willow was left as 'sub' for Miss Calender's class, where she knocked the disk.
Unfortunetly I do not know where Robia LaMorte (Jenny) can be seen next, I hope she shows up soon...