In no particular order.

E E "Doc" Smith - Lensman series ( have been reissued no so long ago so you should be able to find them). They're fun and it's a lengthy series.

C.M. Kornbluth and Fredrick Pohl The Space Merchants,& The Merchants war. Wrote a nice series about if advertising agencies ruled the world. In many ways more timely now than when they were written. Kornbluth has fun short stories along this line about inbreeding and consumerist societies...

A lot of Harry Harrison Novels would likely fit the bill too (I'm mostly thinking of the Stainless Steele rat. )
The Earthsea Trilogy (and the fourth book written much later) recomendation by others here is right on.

A lot of folks mention Enders Game which doesn't sound like what the OP is looking for despite being quite good.....Card's Alvin Maker series probably is and is great fun. The Call of Earth books would be appropriate too, but they are not very good.

While foundation trilogy may not be what the OP is looking for at that age I thought Caves of Steel, Robots of Dawn, and I Robot were awesome!

I'll second the Frank Herbert recommendations but you can go farther afield than Dune to stuff like the Dosadi experiment too.

Tom Swift series - basically Sci Fi Hardy boys. Neither dark nor political but honestly kinda dull.

At the risk of going off topic non Sci-fi thats great at this age is The Hornblower Series.