The capture-mark-release-recapture method provides a simple way to estimate population size. However, when used as part of ecological sampling, this method does not easily allow an opportunity to evaluate the accuracy of the calculation because the actual population size is unknown. Here, we describe a method that can be used to measure the student population of a school. This method allows the calculated population to be compared with a known population, therefore allowing the accuracy of the estimate to be analysed. We also describe some of the pitfalls of the technique and some ideas for further discussion and analysis.