I think it's an appropriate usage of the term. It's an organized effort to abstain from some specific activity, which is a broad but well-understood definition for "boycott." Nations choosing to not send competitors to the Olympics, for example, is widely called a boycott despite it not having anything to do with purchasing a product or service.
The semantic objection is not because of "purchasing" implications, but the intent behind the abstention. Nations boycott the Olympics for political reasons. The term boycott implies collective abstention to influence something outside of the standard transaction. In this case, the intent behind their abstention is inside the standard transaction: they abstained to get good grades.
