I am reluctant to leave the practice league over concerns that I will be 'stuck' in the league I place. Is this the case, or can you earn your way from say Bronze to Silver as you improve?
No, you are not stuck. As you keep playing, you can move between leagues depending on how well you do.
I recommend leaving the practice league as soon as you are ready to start "getting better", since the "real" league gameplay is quite a bit different. First, the game speed is faster. Second, the maps don't have walled-off entrances to protect you from rushes. I'm not sure that spending a lot of time in the practice league will be the best help when it comes to placement matches.