Basically, enchantments that say something like "Reduce magicka cost of Destruction spells by 12%" are additive, so if you have 4 such enchantments that say 25% then 25+25+25+25=100%. Basically you end up with "Reduce magicka cost of Destruction spells by 100%". Max per item is apparently 29%. But I'm not shure if thats true considering the alchemy-enchanting loop that is possible. You can have 4 items with this type of enchantment on at once (helm armor ring necklace) but you need high enough skills, and a good enchantment potion (not sure how the ratios are exactly). Finally, you can get two enchantments on each item, so you can get two different schools.

Personally, I'm looking for a mod that makes it multiplicative, so that 4 25% gear becomes ( 1 - (0.75*0.75*0.75*0.75) ) = 68% reduction. (29% gives around 75% reduction)

edit: also, don't read up about the alchemy-enchantment loop, it will ruin skyrim for you. edit2: the bonus from armor and bonus from perks don't stack btw. basically it's (armor+...+...+armor)*(perk+...+...+perk).