I think it would be easy if u maintain generic one (i.e. 4011), but still listed duties on ur contract should be matched with it, and as well as same duties should go on CIC employment portal by HR, when they submit your LMIA exemption application. include TEACHING stuff related sentence as well in ur duties, if you applying under 4011, that's really crucial.
Secondly, I think you should be OK with existing WP, as condition is for PR, you should maintain legal status in canada, in ur case you have in-hand valid WP, and by the time you will get PR, still that would be VALID, and further you won't need any WP once u have PR...that's my logic....further check with experts on this forum (e.g. Scylla, jes_ON, DelPiero007), they would better assist you, create new post and tag them! they are awesome n always goes an extra mile while being v. supportive n responsive!