The Wall by William Sutcliffe
Joshua is a troubled boy who lives with his mother and stepfather in a divided city, where a wall and soldiers separate two communities, and the rubble-strewn residue of their broken world gives hints of the old life before the wall was built. Joshua discovers a manhole, which leads to a tunnel, which leads in pitch darkness under the wall and across to the other side. Forbidden territory, dangerous territory, violent territory, which a boy like him - visibly different - shouldn't stray into.
An act of kindness from a girl saves his life, but leads to a brutal act of cruelty and a terrible debt he's determined to repay. And no one, no one must find out that he's been there - or the consequences will be unbearable.