And again, "Why aren't more teachers in PAR?"

This is from a 2004 report from the Montgomery County shared accountability office:

Referral Rate for Experienced Teachers

Since the program began in the 2000-2001 school year, 315 tenured MCPS teachers have been referred to PAR.3 District officials have raised the question about why this number is “so low.” Principals were asked precisely this. The answers, varied and instructive, fall into the six categories of response, described below:

Unwillingness to give up on teachers—The majority of principals are interested in helping teachers to improve their practice. They believe, “If I just try a little harder, work a little more with this teacher” things will get better. In MCPS, placing an experienced teacher in PAR sends a signal that the teacher is at risk of being terminated.4 One principal’s comments sum up the views of many: “It’s gut-wrenching. These [teachers] are not strangers. You know their stories. They’re not evil people.”

Concern that referral to PAR reflects negatively on the principal—A number of principals expressed concern that referring an experienced teacher to PAR reflects negatively on them as principals. The teacher being referred is often someone the principal has selected, or helped to select, for the position at the school. Some significant fraction of principals believe that if they refer teachers to PAR, it indicates, “we made a mistake selecting that teacher in the first place” and that “mistake” will translate into something negative on the principal’s evaluation.

Not wanting to roil school waters—Some principals say that knowledge that an experienced teacher has been referred to PAR creates tension at the school. “Other teachers become nervous, or think I’m picking on the teacher.” As a result, some principals admit, they do not refer teachers who ought to be referred to PAR so as to keep peace among the faculty.

The time factor—A number of principals admit that they refer fewer tenured teachers to PAR than might benefit from the assistance because of the time it takes for them to prepare a submission to the PAR Panel. Principals are required to complete two formal observations (down from three). Each of these, by some estimates, requires as long as three hours of the principal’s time. “Sometimes I just miss the deadline because I don’t get the work done in time,” commented one principal whose remarks spoke for a number of colleagues.

Holdover concerns about MCEA—A few principals report they are reluctant to refer experienced teachers to PAR because they are worried, based on history, that “MCEA will make things difficult”. None of the principals interviewed could point to an actual instance in which, since the inception of PAR, the union has attempted to stymie the process. Nevertheless, there is an anticipatory reaction among principals that seems to make some of them more reluctant than they otherwise might be to
recommend tenured teachers to PAR.

Newness of the system, newness of many principals—Finally, PAR is a relatively new system for MCPS. Given the staggered implementation of the PGS, some principals have had a year or two of experience with PAR, but fully one-third of principals began to implement the system just this school year. Compounding the newness of the system is the newness of many principals. Familiarity and comfort with the process likely will encourage greater use of it.

Again, the last pagagraph is simply not supported by the data. The time factor rings home to me. Although, I might call it the "it's a pain" factor.

I will tell you this, and let there be no doubt, if principals were paid extra for great assessment data, more teachers would be on PAR.