Where's the 'rust'?
What colour is it?
Could be, as Andy says, from soldering, but if it's away from solder joints, like inside the bell, and is red, it's the zinc leeching out from the brass by electrolytic action, leaving behind a red copper oxide. Clean it off with a mild metal polish and wax/relaquer as appropriate. If it's green, it's verdigris, clean off with vinegar and then polish.
As for quality, if the Stagg clarinet we bought our son a few years back is anything to go by, it's very basic entry level kit. Price sounds OK, though.