.This is blatant discrimination!

The most common gray areas for gaining experience are located on the edges of the protective barrier, partially covered by the barrier preventing further mutations due to excessive radiation. However, it was still home to various plants and animals that had already mutated way before the barrier was set up. Yet, the plants and animals in there are far less dangerous than the ones outside the barrier that are being subjected to heavy radiation all year round.

So generally , students who are there to hone their skill will enter the gray areas covered by barrier so as to ensure their safety. Only a very few people would dare to step into uncovered gray areas as they contain life threatening dangers that nobody had discovered, so much that you wouldn't even know how you got killed.