We were really fighting for a 3rd director at our CCC middle school but they told us we needed to reach 250 students.
Another school in the district lost their MS assistant because their program dropped below 150. They made it sound like that was a fairly standardized number for admin across that part of the state.
I would come at it from the MS point of view. If 150 is a standard to receive an assistant then you should hopefully be able to get a second director at that campus. If you happen to hire someone who would have a wonderful specialization for your HS program as well then you can just sell that as bonus for the admins.