Starcraft II - Are you stuck in a league once you are placed?
Solution 1:
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.