Middle school students participating in energy-monitoring activities guided by their teachers during 2009-2011 gained (p < 0.05) in content knowledge and became more positive in their dispositions toward STEM (science, technology, engineering, and mathematics). No comparison group data were gathered for this initial study. Activities were replicated with a new group of treatment students during 2013-2014, adding a comparison group not receiving the treatment. Matched pre-post data from 2013 to 2014 confirmed gains in knowledge of environmental science and vampire power (p < 0.0001, effect size = 0.86). Aggregate dispositions toward science, mathematics, engineering and technology became more positive for treatment versus comparison group students (p = 0.023). Gains in STEM dispositions for girls were more positive (effect size = 0.37) than for boys. Implications of these findings are that hands-on, inquiry-based science activities may help increase the STEM career pipeline in ways that can lead to broader participation in STEM careers in the future.