Mex looked at the situation and was delighted by it. If the trainees were the one who threw the glove at them, they do not need to be shown as the group who bullied the weak.
Instead, it was a show of force that would give them pause and allow the kingdom to know that the traditional power of the Grand Sage was still rather effective in lieu of ruling the kingdom. Not to mention, if the King loses, he would have to keep his promise and move all his privilege and governmental powers back to the ministers. This meant that the ministers will owe the Grand Sage a hefty favour.
Not to mention, the Grand Sage could also learn of the skills and magic that enabled the King to create such a powerful isolation barrier. As much as he was the Grand Sage, for the past few weeks, he was not able to decipher the barrier's mysteries nor its incantations. It vexes him. It frustrates him.