I think the analogy is somewhat faulty though; we're not talking about school. Infact the established way of teaching in the Prequels is to have a Padawn and ergo one main teacher. Also you also make a distinction in your sentance when you say "Science Teacher" so you are in fact categorising him.
Plus I think your point would of worked if we were just discussing the OT, but I just don't think your explanation holds water (no offence). I would of been more sympathetic to your point of view if it was just the one mention rather than the too, plus Obi-Wan's statement seemed to indicate he was similar to Luke at HIS age... As such I personally think its a contradiction...
(My apologies for being pedantic on language use; too much philosophy over the years ;-))