我找到了一个解决方案！

I found a solution!

public function info($school, $class, $student, $keyword)
{
    $this->db->where('school.id', $school);
    $this->db->where('class.id', $class);
    $this->db->where('student.id', $student);

    $this->db->where("(school.description LIKE '$keywords' OR class.description LIKE '$keywords' OR student.description LIKE '$keywords')");

    return $this->db->get('info')->result();
}