Musicianship - that's a tricky one, but (obviously depending on the age and instrumental competence) I would expect it to include most if not all of the following:
Good memory for tunes - (not perhaps up to the level of Mozart who allegedly wrote down all of Allegri's Miserere having heard it only once in the Sistine Chapel) - able to play/sing by ear a melody having only heard it once or twice
Good sense of pitch, possibly perfect pitch, but certainly good at pitching intervals or (if not a singer) recognising when a note is out-of-tune with other
Good sense of rhythm - able to recognise the pulse of a piece, clap along in time
Good sense of harmony, able to harmonise a well-known tune by ear on the piano or sing/play a harmony line. Able to hear changes of harmony, recognise cadences
Would probably be able to pick up a new instrument and having learnt a few notes pick out a recognisable tune fairly soon
There are many excellent musicians who are technically very accomplished but struggle to play from memory/by ear, possibly never having been asked to develop those skills. I would classify "excellent musicianship" in a child perhaps as someone who has some or all of those skills almost by instinct. Hopefully that will also include a love of listemeing to and playing music and to lots of practice to improve technical ability, although there can sometimes be a frustration when the perfect sound isn't instantly played on every instrument after 10 minutes!
(PS these are only my thoughts as a long-standing amateur musician)
Re the clarinet problem - it is possible to buy a stand on which to leave an assembled clarinet between playing sessions at home (orchestral players use this if switching between Bb and A clarinets) but this would be risky in a school context. Make sure the cork isn't sticking making it hard to assemble - ask the teacher's advice re cork grease etc.
More detailed musical advice try the Associated Board forums - invaluable
http://www.abrsm.org/forum/index.php?s= ... 3e&act=idx