Primary and Foreign key problem

The following script reports the error ORA-02270. It's regarding the table Students. I don't know if I created right the FKs. Can anybody help me please?

Thank you

create table academic

(employee_id CHAR(8) not null,

	surname		VARCHAR2(20) not null,
	first_name	VARCHAR2(15) not null,
	CONSTRAINT academic_employee_id_pk primary key (employee_id));

create table tutorial


	(academic_id	CHAR(8)	    null,

	tu_day	VARCHAR2(9) not null,
	tu_time	CHAR(5)	    not null,
	tu_room	VARCHAR2(7) not null,
	CONSTRAINT tutorial_tdtimtr_pk primary key (tu_day, tu_time, tu_room),
	CONSTRAINT tutorial_academic_id_fk foreign key (academic_id)
		references academic (employee_id));

create table student


	(student_id	CHAR(8) 	not null,

	en_tu_day	VARCHAR2(9) 	null,
	en_tu_time	CHAR(5)		null,
	en_tu_room	VARCHAR2(7)	null,
	attend_tu_day	VARCHAR2(9)	null,
	attend_tu_time	CHAR(5)		null,
	attend_tu_room	VARCHAR2(7)	null,
	CONSTRAINT student_student_id_pk primary key (student_id),
	CONSTRAINT student_en_tu_day_fk1 foreign key (en_tu_day)
		references tutorial (tu_day),
	CONSTRAINT student_en_tu_time_fk1 foreign key (en_tu_time)
		references tutorial (tu_time),
	CONSTRAINT student_en_tu_room_fk1 foreign key (en_tu_room)
		references tutorial (tu_room),
	CONSTRAINT student_attend_tu_day_fk2 foreign key (attend_tu_day)
		references tutorial (tu_day),
	CONSTRAINT student_attend_tu_time_fk2 foreign key (attend_tu_time)
		references tutorial (tu_time),
	CONSTRAINT student_attend_tu_room_fk2 foreign key (attend_tu_room)
		references tutorial (tu_room));