It doesn't appear to be classical. It means "adorn the white [something]". I'm not sure what the "white" is referring to, though. It would have to be a word of feminine gender in Latin. This is the motto of St. Dunstan's College in London, and it apparently has something to do with the school's coat of arms, which is mostly white (supposedly, I can't seem to find it).
â€œAdorn the white;â€ you are all starting with white shields, and you can adorn the white...Be like Sir Galahad with his white shield on which â€œa bloody crossâ€ was signed, when he had fought and won.
The normal word for shield in Latin, scutum, is neuter. However, there were two kinds of shields that are feminine words in Latin, the pelta, an ancient shield (even to the Romans) made of wicker, and the parma, a round shield which was used by the Romans in the late Empire period. I'm not sure if either of these is implied by albam, though.
in my humble opinion "albam" cannot refer to the coat of arms, in fact scutum is neuter (as Imber Ranae pointed out) and i deem really peculiar that reference was made to two kind of shields unused in classical times. neither "adorn the white" can be accepted because white in latin is album (neuter). since the sentence is the motto of a college i think more reasonable to translate albam with white "toga", the toga of manhood, put on by boys of ancient Rome in their fourteenth year when they left the "toga praetexta" a toga with a large red hem. so i think "albam exorna" could be translated in "adorn the white toga (with the awards you will receive for your committment in studies)
'Albam Exorna' ('Adorn the White') is the motto on the arms of St. Dunstan's College, an English Pubic (Private) School in Catford, SE London. The arms depict in the top left corner an image of the tenth century Bishop, Saint Dunstan. The rest of the shield is white (albam). The point of the motto is that pupils should 'adorn the white' (the undecorated part of the shield) by adding achievements of their own. The school has done its part, now it is up to them to do the rest by realizing their potential. The emphasis is therefore on future achievements, not old tradition. Also individual choice (adorn the white in a way that befits you).
In my humble opinion (full disclosure: I am an 'OD', Old Dunstonian), this is a splendid motto. Yes, it has precedents in Malory and Patrick Hume's 1695 annotations to Milton's Paradise Lost, but it is still a superb motto. Whoever thought it up has more than adorned their white. Even Hogwarts could be proud of a motto like this.