Our research objective focused on monitoring students' mental effort and cognitive achievement to unveil potential effects of an instructional change in an out-of-school laboratory offering gene technology modules. Altogether, 231 students (12th graders) attended our day-long hands-on module. Within a quasi-experimental design, a treatment group followed the newly developed two-step approach derived from cognitive load theory while a control group applied experimentation in a conventional one-step mode. The difference consisted of additional focused discussions combined with noting students' ideas (Step 1) prior to starting any experimental procedure (Step 2). We monitored mental effort (nine times during the teaching unit) and cognitive achievement (in a pre-post-design with follow-up test). The treatment demonstrated a change in instructional efficiency (by combining mental effort and cognitive achievement data), especially for intrinsically high-loaded students. Conclusions for optimizing individual cognitive load in science teaching were drawn. (Contains 5 tables and 1 figure.)