SQL & PL/SQL

I am trying to learn sql, so this is most likely very simple. If there is any more information that you need me to provide, just request it. I am trying to use a trigger that takes the input and puts it into three tables different tables. I am not sure what I am doing wrong as my error message doesn't seem to tell me much. Maybe one of you could get more out of it?

I am using sql developer, 11g and this is installed on Windows 7.

The error reads:

14/32          PL/SQL: ORA-00984: column not allowed here
14/5           PL/SQL: SQL Statement ignored

My tables, sequences and trigger are:

CREATE TABLE students
(
student_id number,
first_name varchar2(50) not null,
last_name varchar2(50) not null,
CONSTRAINT student_id_pk PRIMARY KEY (student_id)
)

CREATE TABLE courses
(
course_id number,
courses varchar2(50),
CONSTRAINT course_id_pk PRIMARY KEY (course_id)
)

CREATE TABLE registration
(
reg_id number not null UNIQUE,
student_id number,
course_id varchar2(100),
CONSTRAINT student_course_pk PRIMARY KEY (student_id, course_id)
)

CREATE SEQUENCE courses_seq
CREATE SEQUENCE reg_id_seq
CREATE SEQUENCE course_id_seq

CREATE OR REPLACE TRIGGER instead_insert_multiple
AFTER INSERT
ON student_info_original
FOR EACH ROW
DECLARE
placeholder number:=1;
course varchar2(50);
firstname varchar2(50);
lastname varchar2(50);
BEGIN
  firstname := SUBSTR(:NEW.student_name, 1, (INSTR(:NEW.student_name,' ')));
	lastname := SUBSTR(:NEW.student_name, (INSTR(:NEW.student_name,' ')+1));
  
  INSERT INTO students VALUES(:NEW.student_id, firstname, lastname);
  
  WHILE placeholder<length(:NEW.courses) LOOP
    course := SUBSTR(:NEW.courses, placeholder, INSTR(', CIS', 0)+8);
    INSERT INTO courses VALUES(courses_seq, course);
    INSERT INTO registration VALUES(reg_id_seq.NEXTVAL, :NEW.student_id, course_id_seq.NEXTVAL);
    placeholder := placeholder+10;
	END LOOP;
END;
/