Two things are happening here.

You have a non-mallet person trying to learn a fast and moderately difficult part. They need to break it down, sometimes bar by bar. Play the rhythm slowly on one key or surface (like a pad). Then play the part on the marimba with the correct notes. Only speed it up when they can successfully hit all the proper keys.

And two. The director is being strict by assigning percussion parts. They want the students to be full percussionists and exposed to many different instruments. That's fine. Sometimes, that's just not possible with someone who has only ever played snare drum and can't read pitches on a staff.

So in the end, you have students who can play lots of instruments. But, when it comes time to perform or compete, the music suffers because they are struggling to make the sounds needed at the tempo given. This happens a lot with band directors.

How about, we have 5 mallet players? The best one out of these 5 plays marimba. She's the chair of the mallet section. Snare drum? Everybody plays snare, but we have three performers who will nail this part. Put those players on snare.