I've found that as a female teacher, a few inexperienced male singers (e.g. those who at 16 have never sung or been used to their instrument since it rather dramatically changed) have occasionally found it difficult to pitch from me, because when I sing their line/harmony at pitch it sounds low on me so they automatically drop an octave. I often use other experienced males to sing their starting note and this helps them to pitch it in the right octave and use the stronger singers in my group to support them. Having your female colleague in the room may well help. The range that you talk about sounds fine for primary singers. Are they using their belt voice? It may be that they have picked up the habit from listening to commercial music and if so you need to stop them doing this and straining. It might be that they are trying to belt their way through it and cannot reach the higher end as their chest belt does not go up to the D. You certainly wouldn't want them to be encouraging them to try and reach a top D in their chest at primary age!