When planning to teach a new selection that includes a tricky rhythm, I suggest teaching the rhythm separately before sight reading the piece.
Place the rhythm on a screen or board in the front of the ensemble. Then, talk and count it through using your preferred counting method. Clap the rhythm with an amplified a metronome beat.
Divide and Conquer
Divide the ensemble in half, and have one half clap a steady beat while the other half claps the rhythm. Switch groups, then try alternating back and forth without stopping. Then, divide the class into thirds, and assign another simple rhythm to be clapped in addition to the steady beat and the rhythm being studied. Again, work this until each group can clap with mastery without stopping between the rhythms.
Next, have students count off in threes, and then assign group 1 the new rhythm, group 2 the steady beat, and group 3 the simple rhythm. Perform this as a class, and have each group cycle through all three rhythms.
These steps could be broken up over a few class days to add to retention and mastery. On a successive day, include the rhythm in a warmup for the ensemble. At this point, you’re ready to introduce the new selection containing the tricky rhythm. Isolate it and ask students to sing or count it aloud before sight reading the piece.
Because the tricky rhythm is not a surprise, but something familiar, you’ve increased the likelihood that every student will experience success.