I'm playing a mage in my current playthrough and although I have been using Destruction magic from the beginning, I noticed that Destruction leveling goes very slowly, especially compared to other magic schools like Illusion and alteration. I've got my Illusion up to 6O in no time and that goes the same for conjuration and alteration. Although i am at level 37 now, my Destruction magica is only at 56, which seems very low, considering I been using destruction since the very beginning, included all the destruction training lessons i took. Did Beth numbed the destruction magic or has destruction leveling always been so slow?

I rarely use destruction. I've never really noticed it leveling slow, no problems here. Perhaps try using the Mage stone for awhile?

I have been using that since the beginning.

I've noticed Destruction and Restoration level slowly. Of course remember, you can only use them in combat. The others can be used anytime.