For Heaven's Sake: YA action & adventure fantasy by David Wickenden
Acting out because of his grief at the loss of his grandfather had sent Adam to Juvenile boot camp. Protecting the inmates from the brutal guards taught him leadership, loyalty, and friendship. Adam and his friends escape boot camp, but find themselves transported to Heaven, which they find under siege. They must learn the skills of a warrior before they enter the portal that will take them to the alien stronghold. There, they must steal back the only weapon that stands a chance of stopping the invasion; the Staff of Moses. If they fail, Heaven will fall and leave the way open for an invasion of earth and the eradication of mankind.