I gotta admit, losing an instrument once, okay....dumb, dumb move. A second time? You have problem, IMO. Sounds like she doesn't understand the value of money. She isn't treating it as a prized possession, or something she respects as costing a significant amount of money. You really need to inquire about her behaviors regarding carrying the instrument. On the bus, it should be in her lap. At school, in her locker or locked away in a band room. She should know it is either in her hand or locked safely away. There are no other options.
I played flute, then clarinet, then oboe for years! Between ages 8 to 18, nobody ever misplaced an instrument. Yes, one girl had hers stolen... During a large scale break in at the family home. She'd just bought a new expensive flute and was devastated.
Really, talk through her behaviors, talk to the band director about options, etc. she needs to learn to respect it as an item of significant value. And if she doesn't, it's either because she doesn't respect money or doesn't really love playing. Sounding a bit harsh here, I know, but losing an instrument a second time suggests more than a dumb mistake any kid could make once.