I just tell myself that those brats won't really listen and might make things much worse if they were told the truth. How? Idk. Maybe they'll get too haughty and think humans are stronger than dragons since the latter picked the generic-looking Iren, who seemingly has a very beautiful royalty friend Miil (amplified by dragon charm), as their representative.
But the boring reason is already mentioned in chapter 2 where Lilisle doesn't want the students to know and change their attitude towards Iren, so that he can judge humanity "as it truly is". All that's left is for the higher-ups to somehow teach the noble kids some humility.