On day 5 of their training, they eventually found out the trick. Since they were not able to use offensive magic to destroy these Spitfire 'Monsters', what they could do is to break out of formation and evade.
And the only offensive that they can have is through the bullets that were bestowed on them by Jin. However, one shot of it would never bring down the Spitfire planes at all regardless of how they tried it. Not to mention, these Spitfire fighter planes were all automated without any humans inside at all compared to the time when the System had to assign a minion to control it.
It seems the planes themselves have a sentience of their own and they could make their own decisions to do things such as retreating away from the trainees or attacking aggressively against them. That was why it was not wrong for the trainees to call them monsters of their own class considering how they were acting.
