An article in the April 2010 issue of the Journal of Research in Science Teaching synthesizes findings on inquiry based instruction in science outcomes for youth.
"Various findings across 138 analyzed studies indicate a clear, positive trend favoring inquiry-based instructional practices, particularly instruction that emphasizes student active thinking and drawing conclusions from data. Teaching strategies that actively engage students in the learning process through scientific investigations are more likely to increase conceptual understanding than are strategies that rely on more passive techniques, which are often necessary in the current standardized-assessment laden educational environment."
Here is a link to the article and accessible PDF.
Citation: Minner, D.D., Levy, A.J., & Century, J. (2010). Inquiry-based science instruction - what is it and does it matter? Results from a research synthesis, years 1984 to 2002. Journal of Research in Science Teaching, 47(4), 474-496.