Unfortunately for Galois theory there isn't anything suitable for high school students,  but the nice introduction is here [Galois theory for beginners, John Stillwell][1], in addition to historic essay in the introduction to the book by Edwards. 

for arithmetics, I think the books by Alan Baker (Theory of numbers) and by G.H. Hardy might be helpful.

for geometry/topology [this  essay by S.S. Chern][1] can provide some motivation for the subject.


  [1]: http://www.jstor.org/stable/2324574