SQL & PL/SQL

Hi,

I want a solution for mutating table error. I am a newbie in oracle.
I'll explain my scenario.
There are two tables TEACHER and STUDENT
both are linked using the field 'tid'. the foreign key relation is given as ON DELETE CASCADE
so if i delete a row from teacher , the corresponding rows in student get deleted, but i want to back up all the students who comes under that teacher, who is getting deleted.
I tried in TRIGGER, but getting mutating table error.

Please help
Thanks in advance
Divya