Section 12. The guardian of a mentally ill or mentally retarded person or spendthrift shall have the care and custody of the person of his ward, except as provided in section twenty-four, and the management of all his estate, and shall give the bond prescribed in section one of chapter two hundred and five; provided, however, that in the case of a guardian of a mentally ill person or spendthrift, the condition prescribed by clause Second of paragraph 6 of said section one of said chapter two hundred and five relative to the education of the ward shall be omitted.