###  The swarm takes radiant damage if it misses the Dexterity save

Because the swarm has its own stat block, like an orc or a dragon or a manticore, it is treated mechanically like a creature when it comes to spell effects and attacks.  It is worth noting what a swarm is and how it is created:   

> They form as a result of some sinister or unwholesome influence ...
> even Druids can't charm these swarms, and their aggressiveness is
> borderline unnatural. (MM p. 337)  

 - This points to a magical (something) being inherent in a swarm that
   keeps it together in this form.   While the MM entry may raise the
   question of whether or not a Swarm is a single creature in your mind, the consistent thing to do is treat it as a single creature since it is statted with its own stat block.      
 - You will note that a swarm of ravens has a series of immunities
   and resistances that a regular beast does not have.   

  > **Damage Resistances** bludgeoning, piercing, slashing   
> **Condition Immunities** charmed, frightened, grappled, paralyzed, petrified, prone, restrained, stunned (SRD p. 391, Swarm of Ravens)  

  That's another point in not treating it like a single raven multiplied X times.  Ravens have no such immunities.  (SRD p. 387, Raven)

 - In aggregate, the nature of "swarming" makes a bunch of single
   creatures act collectively and thus pose a greater danger to a party
   than a few rats, piranhas, ravens or bats. (Consider that a squad of
   soldiers or goblins is far more dangerous than a single soldier, but
   since there is no "swarm of goblins" stat block that analogy ends
   there).

 - A Swarm does more damage in "swarm" form by a significant amount: the
   swarm does 2d6 and a raven does 1.  for the purposes of melee and
   movement (in formation, however chaotic) a Swarm is should be treated
   as a 'creature' even though the Swarm is made up of a bunch of
   smaller creatures.

Once you adopt the "keep it simple" approach, the answer to your question is very clear:  

 1. Roll the d20 against the spell DC for the caster  
 2. Add +2 for the 14 dexterity score of the swarm,
 3. See if the swarm is damaged or not, based on missing or making the save.