I'll try and be vague so I don't identify the school/HT.
HT doesn't like the subject and has openly admitted that if it wasn't a core subject, she wouldn't have it in the school curriculum. She overrules the HOD on recruiting issues and doesn't choose the best candidate for the role. Then the new teacher has an adverse affect on the results. She has reduced the timetable each year for the subject. She publically berates the HOD for worsening results and tells the whole school this department is the worst in the school. The entire school has been refurbished except this department. She is HT at another school too where she has been parachuted in to get it out of special measures and spends all her time there. Teachers she doesn't like have been put on capability so she can get rid of them.
It used to be a good department with some outstanding features. A new HOD had been recruited and it had some outstanding teachers, students were beginning to take a real interest in the subject and get good results in Y10. But bit by bit this has been undermined and taken apart by the HT.
As an outsider, it seems like a clear case of sabotage as her decisions have been so damaging for the department and make no logical sense whatsoever. The SLT don't stand up to her and anyone that questions her actions is put on capability.
I genuinely don't understand how this is permissible.