Are you talking about just having a list of instruments you have, with information about each instrument (Brand, Model, Serial #, etc.)? I’ve had good success just using Microsoft Excel to do this. I was also one of the beta testers for ischoolmusic.org which has a place for you to keep an instrument and uniform database. I haven’t been on the site much lately though (it is still kinda buggy), but I remember it would let you actually assign the items in your inventory to your students (assuming they had an account on the website as well), and would also let you mark if an item was out for repairs.
I’m not really sure what the Charms system is.