The instructional lab setting has been found to be dominated by prescribed tasks and pre-prepared lab kits. This was explained by teachers' need to guide students to simultaneously progress through a lab curriculum, which prompts them to standardize the lab experience. Nevertheless, prominent professional associations have persistently called to better represent experimental research practices in the lab, and to grant students more agency in the experimental process by orienting them towards more open-ended lab experiences. This paper reports a lab activity designed to advance students' agency in the practice of experimental design, in a setting governed by a high-stakes national matriculation exam. Three hundred teachers of advanced level high-school physics experienced the lab activity in a national network of professional learning communities (PLCs). The activity was anchored in an experiment to determine the relationship between the current through a battery and its terminal voltage. It was designed to problematize students' considerations underlying the choices of the location of the voltmeter and the measuring scale of the ammeter, and the possible implications for the validity of the experimental results; e.g. control of the variables, as well as the range and the accuracy of measurements. Teachers first performed the lab activity as learners, discussed it in the PLC meeting, and finally reflected on their experience. Individual responses to the lab worksheets and the reflections were analyzed. Initially, teachers' considerations did not portray key aspects related to the validity of the experimental results, such as how design choices related to the location of the voltmeter and the ammeter measuring scale impacted the accuracy and range of the measurements and the control of variables. The teachers were highly engaged in the peer discussion in the PLC and found the lab activity valuable in raising students' awareness of important considerations in experimental design.