The worst thing is the mixed lighting that you can't control. I'm imagining a school library to have big windows letting in a lot of sunlight, and overhead flourescents filling in. Unless they're daylight-balanced flourescents (they're not), it'll be impossible to get perfect WB. Try auto and custom and see what gives you the best results.
On the other hand, the light may be really low. If you're only printing 4x6's, don't be afraid of ISO1600. The noise is barely noticable at that level. Open up your 43 Ltd and fire away.
If the light is so low that you have AF problems, try this. If you're shooting multiple shots at the same distance, hold down the AF button on the back. It will focus once and will hold that focal point as long as you hold the button.
Alternately, you could throw everything to auto and shoe the Metz. I would advise against direct flash, though. If you can get a Demb bouncer, they're fantastic. If not, foam works great too, it just looks a little jury-rigged.
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