The false prince - Jennifer A. Nielsen

If you love the danger and sword-fighting of MERLIN, you'll like
this!
In a discontent kingdom, civil war is brewing. To unify the divided
people, Conner, a nobleman of the court, devises a cunning plan
to find an impersonator of the king's long-lost son and install
him as a puppet prince.
Four orphans are recruited to compete for the role, including a defiant
boy named Sage. Sage knows that Conner's motives are more
than questionable, yet his life balances on a sword's point - he
must be chosen to play the prince or he will certainly be killed.
As Sage's journey continues, layer upon layer of treachery and deceit
unfold, until finally a truth is revealed that proves more dangerous
than all of the lies put together.