I have a new name for the piano in my classroom - it is being re-christened the "assessment center".
Music teachers are constantly assessing. The assessment/instruction cycle in a music classroom is so short that it is almost imperceptible. It is part of a music teachers's DNA that what you hear (assess) instantly guides what you say (instruct). Add this to the fact that many of the procedural skills a choral music teacher assesses are things humans usually do unconsciously (breath support, vowel shape/tone, diction, etc.) and you have a quagmire of assessment for instruction. This is the challenge that music teachers somehow love - finding the perfect sound.
In my elementary classroom, the students first walk in and stand around the piano. We do warmups together - for the same reasons that any choir would do warmups - we work on beautiful sound. Sometimes we sing in small groups or individually. This is our pre-assessment.
When I am teaching a song, we usually use the CD's that come with our textbooks or other resources (Activate, Music Express, MK8). But when it's time to see how the class is progressing - back to the piano! We make this a big deal - can you sing/play it with the piano?? These are our formative assessments.
Finally, when we have a song down pretty well, it's time to record it and publish it to our website. Where do we record the song? Around the piano! This is our summative assessment. Is everything we put out on the internet perfect? No. But at the elementary level, the focus is on process, not product. And letting the students listen to the recording lets them assess themselves, too.
Hope you love your new assessment center.