I wouldn't have thought so. I would look at the characteristics of the piece that make it of that standard and apply those to the pieces that your students want to sing. I do Edexcel and it lists things to look out for in pieces that make them 'easier', 'standard' and 'more difficult' (wide range, advanced techniques, tricky rhythms etc). It is also about getting the balance right, a student doing a piece that is difficult enough to show technique, but not so difficult that they cannot interpret it accurately and communicate it effectively. You could always post on here what pieces they are thinking of?