My district supports me giving grades for concerts, but I do have a rubric based on concert etiquette that I use to justify my assessment of their performance. I also have an alternative assignment for those kids that miss the concert due to illness, etc. They have to come in during their recess (I teach 1-5) and sing the concert for me, as well as complete a two page take home test over concepts we learned with each of the songs, and concert etiquette. They also have to write a paragraph about why they missed the concert and have a parent sign it.
I make sure everyone is aware of the guidelines (parents and admin. included) and that performing live is much different than just learning songs in class. I tell them, this is our TEST for the grading period! We don’t do any other formal assessments in the weeks/months leading up the concert.
That being said, I know that in some states you cannot give a grade for after school activities (such as concerts). Having a supportive admin. helps most people bypass that ruling. Good luck!