I think the shields ought to have some skills specific to the shield and not part of any skill school.
For example, Battering Ram ought to require a shield to use, but then have a shorter cooldown. Melees already have hammer blow.
Repair skill, costs 1 AP and can be used in the fight to 'repair' your shield giving you back the lost shield armour that you have taken so far, assuming shield armour is taken first.
Magic Armour Steal, costs 2AP and steals magic armour, but doesn't let you go over your magic armour max of course.