Little Soldier by Bernard Ashley
When Kaninda survives a brutal attack on his village in East Africa he joins the rebel army, where he's trained to carry weapons, and use them.
But aid workers take him to London, to a new family and a comprehensive school. Clan and tribal conflicts are everywhere, and on the streets it's estate versus estate, urban tribe against urban tribe.
All Kaninda wants it to get back to his own war and take revenge on his enemies. But together with Laura Rose, the daughter of his new family, he is drawn into a dangerous local conflict that is spiraling out of control.