Can you do an all-school assembly?

Yes, however I don’t usually recommend it unless your school has only primary or only intermediate grade levels.

I find it’s best to provide separate assemblies for primary and intermediate students. The writing lesson I do with primary grades is simpler than what I do with older kids. In a K-5 or K-6 school, I can do an all-school assembly, but I have to leave out the writing portion of the program and focus more on the performance and the Q&A.