I assessed in my music classroom long before it was required, over 30 years ago when I first began teaching. Authentic assessment is how a teacher can evaluate the effectiveness of instruction. I also was part of the team that developed the elementary music assessment for our district.
I used observation and performance assessments with all students and one written assessment per grading period with 2nd-5th grades. Because we were required to turn in grades (E, S, N), I felt they had to be based on something tangible. I refused to give all students an "S" or "E" with no data on which to base that.
I used an Excel spreadsheet and then merged/generated a "Report Home" in Word with a Skills and Behavior grade. I tried to assess at least 6 skills every grading period. Parents loved it and responded with a very nice budget for me to use for the program. A nice residual effect! One parent even complained to the principal that he was receiving more information about how his child was doing in music class than in the regular classroom.
Assessment is one of the best ways to educate the other adults—parents, admin, board members, etc.— that music is as viable and important in the curriculum as math and science. I realize this is not a popular stance, but I feel very strongly that if we want music to continue to be included in the curriculum, we must teach with sequential plans based on the standards and assess accordingly. Students love the related arts classes because teachers have been teaching and assessing the way it should be done for many years. We just didn't take it to the next level of recording and reporting.
Assessing does NOT mean you are "teaching to the test" and turning your classroom into another assessment-based subject. It does mean that you have to be creative and demonstrate that measurable learning happens in the music classroom. And it is very important to be able to back that up with data. Assessments must be fun and non-threatening. Students should look forward to sharing what they have learned. Go the extra mile. It's worth it!!