Hi there,
I didn’t deviate too far from the given themes, and chose words related to those themes but with increasing word-length. I didn’t want to make them too hard, so I didn’t change case or add in odd characters. That might be useful for even harder versions (extreme, or fiendish!).
Easy: Local Library: child, adult, books, table, dates (word length 5)
Medium: School: physics, chemistry, biology, english, history (word length 7-8)
Hard: Pentagon: continental, biological, espionage, intelligence, government
(word length 10-12)