Nancy O'Neill Breth, in her presentation to the 2005 MTNA National Conference in Seattle, argues for the importance of finding appropriate collaborative repertoire at an early stage in a pianist's development:
Typically, piano students begin playing chamber music only after they have studied for years and are ready to attempt Mozart or Beethoven trios. Yet practicing with other people can be a lot more fun than practicing alone. When pianists start playing chamber music, they gain an additional reward for their hours of solitary practice. And in turn the ensemble playing deepens their musical skills and understanding.
She goes on to list high-quality repertoire at various levels (although only of the chamber music variety rather than vocal), as well as info on publishers and websites.
I disagree with Breth's statement that "the difficulty of finding interesting chamber music pieces appropriate for early level players is an important reason that teachers hesitate to introduce young students to ensemble playing." No need to be diplomatic--the reason that most piano teachers don't introduce their students to chamber or vocal collaboration at an early age is that they don't know of its importance or simply don't care about introducing it, content to have their students play solo repertoire only. The teachers who recognize that playing with instrumentalists and singers is important at a young age will find appropriate music, get an ensemble together, and have their students reap the benefits of these activities.
Nevertheless, Breth's MTNA paper is part of a growing corpus of information on the importance of finding ways of introducing pianists to ensemble playing, as well as the repertoire and process to make it happen.
The Young Collaborative Pianist Part I
s55ael in St. John's writes about setting up a studio with the long-term goal of introducing collaboration:
Having only set the studio up recently (less than 18 months ago) I don't focus on yet on having students work with each other due to the majority of my students being 8 years of age or so and in the first two years of playing. However, when they bring their pieces back to me prepared well, we *always* perform it as a duet. I have found the Faber Piano Adventure series have beautiful and creative duet arrangements. I'm looking forward to when the studio has developed enough in skill level and number of students to start pairing them and I will certainly use my contacts from all my accompanying to pair instrumentalists/singers of other teachers. And lastly, it makes a huge difference, I strongly believe, for the piano teacher to be an active performing pianist.