I think I've found an error in the Apprentice book on page 10. In the paragraph beginning by Both the Skilled Parens, the teaching source quality for a skilled parens is of 18 and in the next paragraph of 19. Wich is the good ?
The correct value is 18.
A teaching Source Quality is Com + Teaching +3 + bonus (core book p164)
If teaching only one student at a time (which is always the case when teaching Arts), you get a bonus of +6. So the SQ (not including any special virtues or flaws) is Com+Teaching+9.
Apprentices p10 says SQ of 18+ or (Com+Teaching) of 9+. The latter would give the former.