Ok this epsiode is called teachers pet and here is the summary

Jo has become close friends with her English teacher, Gail, even inspiring Jo to think about becoming a teacher herself. However, Jo is disappointed to learn that Gail is leaving Eastland presumably for a job that pays more but the real reason she's leaving is nothing Jo ever expected.

WHAT IS THE REASON SHE IS LEAVING? I HAVE TO KNOW!!!!!!

because she was dieing :( that episode was really sad

Thanks :)