Minha resolução do desafio desta aula

CREATE OR REPLACE FUNCTION create_instrutor () RETURNS TRIGGER AS $$ 
    DECLARE
        wage_average DECIMAL;
        low_paid_instrutors INTEGER DEFAULT 0;
        total_instrutors INTEGER DEFAULT 0;
        wage DECIMAL;
        percentuality DECIMAL(5, 2);
    BEGIN
        SELECT AVG(instrutor.wage) INTO wage_average FROM instrutor WHERE id <> NEW.id;

        IF NEW.wage > wage_average THEN
            INSERT INTO log_instrutors (description) VALUES (NEW.instrutor_name || ' wage is upper the average.');
        END IF;

        FOR wage IN SELECT instrutor.wage FROM instrutor WHERE id <> NEW.id LOOP
            total_instrutors := total_instrutors + 1;

            IF NEW.wage > wage THEN
                low_paid_instrutors := low_paid_instrutors + 1;
            END IF;
        END LOOP;

        percentuality = low_paid_instrutors::DECIMAL / total_instrutors::DECIMAL * 100;

        IF percentuality >= 100.00 THEN
            SELECT instrutor.wage INTO NEW.wage FROM instrutor ORDER BY wage DESC LIMIT 1;
        END IF;

        INSERT INTO log_instrutors (description) VALUES (NEW.instrutor_name || ' receives more them ' || percentuality || '% of the instrutors cast.');

        RETURN NEW;
    END;
$$ LANGUAGE plpgsql;

Observação: Para impedir o insert de um instrutor que receba acima da media com percentual maior ou igual a 100% basta trocar a linha :

SELECT instrutor.wage INTO NEW.wage FROM instrutor ORDER BY wage DESC LIMIT 1;

Pela seguinte linha:

RETURN NULL;