This paper reports an investigation of the performance of school students at age 15 on written diagnostic questions involving the prediction of meter readings in simple electric circuits with parallel branches.  Results show that these students tend to perceive such questions in resistance-current terms, with a primary focus on resistor addition, rather than in voltage terms, with two loads being driven from the same source.  Common errors in predicting voltages across parallel branches suggest that few students use a mental model of solving parallel circuit problems but instead attempt to solve problems by mechanical use of the V=IR equation.  Implications for the teaching of electric circuits at this level are identified.