Push in is actually a model for meeting the needs of different learners that is used quite effectively when addressing low-performing students. There is no obvious reason it should not be effective with certain high-performing students as well. But like all forms of differentiation, there is usually a point at which the range of students is too wide for the extremes to benefit.
As a blanket principle, placing children of different ability levels in different classrooms full time presents philosophical challenges to our ideal of a caste-less, multicultural, diverse society. Which, of course, is one of the ongoing tensions of gifted programming in general.
