Keep them on the baritone part. You really think level 1 will have separate low brass parts? If the trombone part does do something significant and separate, point it out in the discussion time. "There is a part here in the trombones that you will not hear."

Did a 6A varsity with no trombones one year (long story). As I recall, there was a short spot in the SR where they were exposed, but it wasn't the only thing happening. I pointed it out as being missing and that it would sound thin there (for the kids to not freak) and went on. It was fine.