[Sebastian is neither staff nor student, thus he has no idea why Sherlock is standing outside the school of all places, on this lazy Saturday morning. He pauses as he passes by, and then heads over.]
Good morning, Mr Holmes. I had thought the school here had no Saturday lessons?
[As for what Sebastian is doing out and about so early in the morning? Who knows. He has his arms full of various fabrics - some thick and heavy, some light and more appropriate for summer - in muted colours. No such thing as a lazy Saturday for him.]