Between classroom design
In a between classroom design, classes are randomly assigned to conditions. From a strictly experimental viewpoint, this design is rather detrimental: In comparison to a within classroom design, internal validity is reduced since differences can occur not only due to experimental variation and 'statistical chance, but also due to prior group differences (see Cook & Shadish, 1994). Therefore, some scientists argue that data should be analysed at the group level, yielding smaller sample sizes and reduced statistical power.
However, given certain circumstances, a between design can be the best choice to make. For instance, when study conditions are rather intrusive and the chance high that participants will get to know about the other conditions' treatment, the study's validity in a within classroom design is reduced. In this case, a between design is preferable.