Section 21. Any pupil of a private trade school as defined in section twenty-one A, who is defrauded by a misrepresentation made by an officer or agent of such school, or by any advertisement or circular issued by it, or by any person, firm, association or corporation, who sells textbooks to the said school or to the pupils thereof, may recover in contract from such school or person, firm, association or corporation three times the amount paid by him to such school or person, firm, association or corporation.