aside from the usual demographic profiles which is very subjective, other measures or process to set equality between the experimental and controlled group
I am not sure what you mean by "equilibrium" with pupils as a subject. Do you mean relating probabilities and staistics to subsets of a population of sudents to predict sample size distribution based on the known larger group distribution? This is quite a bit different to "more classical "equilibrium" involving chemical reactions and gibbs fre energy but you can use this type of a technique to demonstrate some of the concepts involved in thermodynamics and compare it to probabiliies of selecting objects from a known larger population.
Sir Rick Manner, i mean of equilibrium is the "equality" established between the controlled and experimental group, since the study will be dealing with pupils, and with the notion that no 2 individuals are alike...the previous approach will be to use the age and sex only