This paper presents a new semi-quantitative metric, Green Star (GS), for evaluation of the global greenness of chemical reactions used in teaching laboratories. Its purpose is to help choose the more acceptable reactions for implementing Green Chemistry (GC) and to identify suitable modifications of protocols to improve the greenness of the chemistry practiced by students. GS considers globally, in principle, all the Twelve Principles of GC. The metric consists in the evaluation of the greenness of the reaction for each principle by pre-defined criteria, followed by graphical representation of the results in an Excel radar chart – the fuller the chart, the higher degree of greenness. To illustrate the construction and the scope of the metric, a case study is presented – the iron(II) oxalate dihydrate synthesis performed under several sets of conditions to pursue the implementation of greenness.