You can do whatever you want with a march in regards to rescoring.

If it's a major rescore (like no brass/perc until letter X), you should indicate this in the scores for the judges.

If it's a minor rescore like omitting WW players that can't play a few measure or a section, just have them not play. You could also give them an easier part in the score like a brass instrument part for that section.

What matters the most is that you are having the kids execute the parts with as much musical & technical perfection as possible.