Here's a plot of student growth suburban RI elementary schools. It should be a fairly homogeneous group, similar sized schools, in most cases the same teachers teaching both reading and math to the same sets of kids.
The interesting thing here is that if you flick the switch from reading to math and back, the dots move from side to side quite a bit, indicating changes in growth between subjects.
This is not particularly surprising, unless you think "teacher quality" is some kind of transcendent property with a strong affect on student achievement and student growth percentages is a great way to measure it. In that case, you'd expect the same sets of teachers and kids to generate similar growth across different subjects.
Also, from a technical point of view, this effect would be harder to see without the animated data graphics, but on the other hand, the dancing dots may be exaggerating the actual effect size.