Table is mutating, trigger/function may not see it (stopping an average grade from dropping below 2.5)

use this statement inside DECLARE, it will work.

pragma autonomous_transaction;

First you need to read about triggers, mutating table error and compound triggers: http://docs.oracle.com/cd/E11882_01/appdev.112/e25519/triggers.htm#LNPLS2005

Your trigger is AFTER UPDATE OR INSERT OR DELETE. Means if you run UPDATE OR INSERT OR DELETE statements on this table, the trigger will fire. But you are trying to update the same table again inside your trigger, which is compl. wrong. This this is why you are getting the error. You cannot modify the same table the trigger is firing on. The purpose of trigger is to fire automatically when table is updated, inserted or deleted in your case. What you need is some procedure, not trigger.

I think you can fix this by rewriting this as a before trigger, rather than an after trigger. However, this might be a little complicated for inserts and deletes. The idea is:

CREATE OR REPLACE TRIGGER stopChange
    BEFORE UPDATE OR INSERT OR DELETE ON taking
    REFERENCING OLD AS old
    NEW AS new
    FOR EACH ROW
DECLARE

grd_avg taking.grade%TYPE;

BEGIN
    SELECT (SUM(grade) - oldgrade + new.grade) / count(*)
    INTO grd_avg
    FROM taking
    WHERE studentnum = :new.studentnum
    AND schedulenum = :new.schedulenum
    AND semester = :new.semester;

    IF grd_avg < 2.5 THEN
        new.grade = old.grade
    END IF;
END;