It seems I've come across a narrative inconsistency. Spoilers ahead!
In one playthrough, the princess is revealed to be the Demon Lord and states that such title and power becomes inherited only after vanquishing the previous Demon Lord. Gallafrey then becomes the Demon Lord. Meanwhile, Gallafrey can murder the princess in another playthrough, albeit without knowing that she's the Demon Lord, yet doesn't become the Demon Lord.
For around twenty minutes, I was stuck at this stage too and had attempted it twice after restarting in the hopes of correcting some bug. The items I had left to remove were: blue paper airplane, green desk, red chair, orange desk, navy desk, pink desk, and purple book. However, I just finished the classroom after referencing Mobile Gamer Dad's video on YouTube. For some items including the purple book, they must be aligned at a specific angle for them to disappear. Hope you're able to move forward like I was able to!
