it's been a long time since you followed up on this, and you've changed which tables you are referring to.
did you fix the issue on MODEL_CODE and now have the same mutating issue on another suite of tables?
can you show us the DDL of both the STUD table, and the trigger you created to try and update the LIB table?
there's lots and lots of examples you could modify if you are stuck on syntax, so you've got to provide a lot more detail if you want help from a forum.
Search q=oracle+trigger+update+another+table
Lowell