There is lots of research for working with small groups of children to teach them new words. For a review of these studies, see these two papers:
MARULIS, L. M. and NEUMAN, S. B., 2010, The effects of vocabulary intervention on young children’s word learning: A meta-analysis. Review of Educational Research, 80, 300-335.
STEELE, S. C., and MILLS, M. T., 2011, Vocabulary intervention for school-age children with language impairment: A review of evidence and good practice. Child Language Teaching and Therapy, 27(3), 354-370.
Prof Victoria Joffe has also produced a resource for supporting teenagers to learn new words in small groups. This detailed programme is available to buy from the publisher Speechmark. She also has a website which gives details of the project which developed and evaluated her vocabulary enrichment programme. This can be found here. The Enriching Language and Communication in Secondary Schools (ELCISS) project involved a collaboration between a speech and language therapist, teaching assistants and teachers and it was initially delivered in London.