I read on the skyrim wiki that conjuring atronachs etc only levels up the conjuration skill if you cast while in combat. Does this mean that if I am sneaking around an enemy - so not technically in combat - and then conjure something to attack them, my skills won't increase?
As long as your summoned/reanimated being attacks something before is vanishes/disintegrates it will contribute towards experience gain. See the following from this UESP article:
When summoning creatures, you don't have to be engaged in combat at the time of casting to gain experience in Conjuration, as long as you encounter a hostile enemy before the summons vanishes
To further elaborate on that quote, it seems that neither you or your summon have to engage the enemy in combat, simply being spotted followed by the enemy pursuing you should be sufficient.