Honestly? One of the perks of being a HoD is to pick the bits you want to do. Obviously, this should be done with some idea of balance and fairness. I would be surprised if you went to a school where HoD did not pick the KS4+5 that they wanted and left themselves bottom sets and KS3 alone. Sometimes this is necessary, but you would be naive to think that it would be radically different in a different school.