The Restoration spells that are avaliable when joining the Dawnguard deal sun damange. The damage cuased by theses spells levels up the destruction skill. This is not mentioned in any of the relevant sections. I am not confidant to edit the pages so I am mentioning it here.
The spells are; Stendarr's Aura, Vampires Bane & Sunfire.
I don't have Dawnguard, but looking at the wki page it seems that they're restoration spells despite having a destruction book symbol. If on PC, type advskill restoration 10000 and see if the damage changes.
They are under the Restoration school and benefit from restoration perks. Casting them also levels up Restoration however I have found that the damaged caused by the spells also raised my Destruciton skill. I have only focused on the Restoration school and only have the Flames destruction spell, that I never use. My Destrustion skill has risen considerably while using the spells (mainly Stendarr's Aura) and fighting undead.