Why is it that when I play the violin my bow keeps hitting my leg?

Try putting your right leg back when you play (I assume you are talking about when you sit and play). The other problem may be that you are tilting the violin too much on your shoulder, try a shoulder rest that lifts the treble side of the instrument more.

Sitting on the edge of your chair and moving your foot back, as already suggested by others, are good tips. If you are tall and hitting your thigh because you are low to the ground in proportion to your body, you can move not only more to the front edge of your seat but also to the right edge, where you can drop your knee out of the way. If you are short and are hitting your thigh (my problem) you probably cannot drop your knee, so rotate your hips and legs ever so slightly to the right and move your legs further apart so when your right hand comes down it lines up more with the space between your legs rather than your thigh. Make sure you don't rotate so much that you are twisting your spine, since that can cause all sorts of other problems. You just want to get that leg out of the way while maintaining a stable foundation.

Happens to me all the time before, but here are two solutions

1) Stand up and play

2) Change your posture. First, sit on the EDGE of the chair. Then, try moving your right foot backwards slightly or tucking your right leg under it. Make sure the upper half of your body is exactly like when you're standing, with a straight back and neck.

Your teacher will show you a posture where this doesn't happen. Listen to him.