In case you haven't heard enough reasons why a biplane isn't a good trainer, here's another that I don't think has been mentioned yet. The two wings sometimes lead to confusion about the orientation of the plane. Even experienced pilots sometimes get confused about the plane's attitude due to the second wing. This can be minimized by flying close in and lower to the ground, but these are not things a beginner should be doing. The high wing layout of the typical trainer makes the orientation more obvious.