if there r enough teachers available in school then an A.T. with condensed course should'nt b allowed to take classes in H.S.sections.i think you'd find it in mannual,but why did you mention that the candidate is male nd of 55 yrs? is'nt it unnecessary?
on other hand sometime school authority finds a candidate much more eligible to send to higher classes despite his/her qualifications.
More over if you find it illegal nd harmful or autocracy of authority then you can contact the related D.I. office.
i don't have any G.O. no in my hand right now ,other forum friends will provide you(hope so).
remember situation varies from school to school.