Assessment is never cut and dry in music ensembles in schools like bands and choirs. The ability to track musical progress comes in a number of forms and it depends on how you view music at a philosophical level. There are two main thoughts: one is that music is an aesthetic object and the other is that music is an action performed by humans. I realize that this is a gross oversimplification of the philosophies of Bennett Reimer and David Elliot respectively. They are often refered to as 'aesthetic vs. praxial" philosophies. If you have the free time and really want to dig into it, here's a good breakdown:

http://maydaygroup.org/wp-content/uploads/2014/08/Regelski-Implications.pdf

Any way, these two philosophies are reflected in the state assessment standards. Some are very reflective of Reimer's aesthetic philosophy in that they treat music as an object to be experienced and ask for students to describe elements of music like one would describe literary devices in poetry. Here is a link to one such assessment and the state standard surrounding sensory elements:

http://www.isbe.net/ils/fine_arts/music/stage_I/Mu25AI.pdf

In contrast, here is a self critique form for a student to use when they listen to themselves play a part into a recording device, something reflective of Elliot's praxial philosoohy:

http://www.isbe.net/ils/fine_arts/music/stage_I/Mu26BI.pdf

So the big question is, which is a more effective tool to measure music learning in a large ensemble like band, orchestra, or choir? Is it both? Ensembles provide a unique experience of performance that is difficult to assess simply because every individual's contribution is different. For example, here is a short piece that is commonly played by high school bands:

So how would you assess that? Individually? As a group? Do you have them play their individual parts in a recording device? Live? What about the tuba player that playing a much less technically demanding part than the trumpets? Can you give a written test about the elements of that piece?

In music assessment, the answer is "Yes." I've seen and done all those things. Teachers assess based on whether they believe that music is an action or an aesthetic. Arguably, aesthetic is easier to assess because we ask about the music as an object and there are more objective correct answers, but it also misses the praxial side of the equation since music is performance. But what are the merits of one over the other aside from ease of use? Should music assessment be so based on right and wrong answers?

I'll avoid the philosophical rabbit hole and leave you guys to further explore it.