Is that an assessment to fire teachers?
It's not his goal. It's intended to develop the team: it identifies strengths, defines training priorities and creates a common standard. How to use results is decided by your institution.
Do the teachers know they'll be observed?
Yes. We recommend that we communicate the process with advance and share the rubric: the evaluation works best when the teachers understand it as accompaniment.
Can they evaluate group and band or choir classes?
Yes. The rudder conforms to the format: individual class, group, ensemble, band, choir or early childhood musical initiation.
How much should we repeat the assessment?
Many institutions do so once an academic period or an year to measure progress with respect to the development plan.