If I enter the list from the math screen, the items show up on the other screen.
Homework_planner_and_manager_with_youngeun.aia (13.5 KB)
And the question is ...? 
You made separate Screens for each subject, with exactly the same components and blocks.
Your TinyDB components use the same NameSpace (TinyDB1), and you refer to the same tag in each screen for your list of assignments.
That's why your assignments leak from Subject to Subject.
The easiest Band-Aid is to go through each subject Screen and change the NameSpace of the TinyDBs to match the Subject. That would keep the values separate.
A smarter approach would be to stick to just one Screen, and select Subject from a List you could edit, then use the Subject name as the TinyDB tag where you put the assignments for that Subject.
That would allow unlimited Subjects.