Creating First Trigger Postgres

CREATE TABLE SCHOOL (
    ID          SERIAL      PRIMARY KEY,
    First_name  VARCHAR(30) NOT NULL,
    Last_name   VARCHAR(30) NOT NULL,
    Jurusan     VARCHAR(20) NOT NULL
);

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CREATE TABLE SCHOOL_CLASS (
    ID          SERIAL      PRIMARY KEY,
    Absen       INTEGER     NOT NULL,
    First_name  VARCHAR(30) NOT NULL,
    Old_Jurusan VARCHAR(20) NOT NULL,
    New_Jurusan VARCHAR(20) NOT NULL
);

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CREATE OR REPLACE FUNCTION Jurusan_changes()
    RETURNS TRIGGER AS
$BODY$
BEGIN
    IF NEW.Jurusan <> OLD.Jurusan THEN
        INSERT INTO SCHOOL_CLASS(Absen, First_name, Old_Jurusan, New_Jurusan)
        VALUES(OLD.ID, OLD.First_name, OLD.Jurusan, NEW.Jurusan);
    END IF;

    RETURN NEW;
END;
$BODY$

LANGUAGE plpgsql;

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CREATE TRIGGER Jurusan_change
    BEFORE UPDATE
    ON SCHOOL
    FOR EACH ROW
    EXECUTE PROCEDURE Jurusan_changes();

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INSERT INTO SCHOOL (First_name, Last_name, Jurusan)
VALUES ('Muhammad Dzaky', 'Bahrul Ilmi', 'IPA');

INSERT INTO SCHOOL (First_name, Last_name, Jurusan)
VALUES ('Muhammad Rafi', 'Bahrur Rizki', 'IPA');

INSERT INTO SCHOOL (First_name, Last_name, Jurusan)
VALUES ('Shafa Nurul', 'Ilyanti', 'IPA');

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SELECT * FROM SCHOOL;

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UPDATE SCHOOL
SET Jurusan = 'Kedokteran Gigi'
WHERE ID = 3;

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SELECT * FROM SCHOOL_CLASS;