i am kinda confused about what just happened but ok
Teacher didn't bother reading the teaching manual, and skipped right to practical exercises. Because she's a bit of a muscle-brain.
For practical exercises, she conjured a separate subspace and pulled the students inside. That way, they'll have plenty of space, and the school would be safe.
She started with a mock battle, and ended up fighting the cute Sage. In theory, higher-tier magic is supposed to beat lower-tier magic, so the Sage being limited to 5th tier is supposed to be a serious limitation. Not for the sage, who can cast spells that cast spells. With the sheer number of spells, the teacher's spells are overwhelmed.
They've been escalating things for a while, until the teacher used a black flag and summoned an undead army. And the Hero and the Sage immediately recognized that flag. It's the Hero's flag from his previous life. Now they want answers.
The talk was interrupted when the school's director called and started complaining. Apparently, the subspace magic tool was not rated to withstand epic battles like the one they were about to start.