Yes, the mysteries of Adobe Student/Teacher pricing are beyond me.
Fortunately, I bought into the system way back in the days when Pagemaker was available. Through incremental upgrades, special offers, etc., I finally got to the point where I have the entire Design Premium Suite and can just directly upgrade from that.
I have heard that there is a path to convert Student-Teacher editions to regular editions, but I don't know if that is urban legend or not.