The height a dish needs to be to clear an obstacle depends on the elevation angle of the satellite you're trying to hit. By dividing the height of the obstacle you're trying to clear by the distance your dish will be from the obstacle, and comparing it to the tangent of the angle of elevation for the sat in question you can figure it out. This assumes your dish is sitting on the ground. When mounting it on a pole, you can subtract the height of the dish from the height of the obstacle in your calculations.
For instance (dish on ground):
For the school, 30/105 = .2857
For the fence, 6/55 = .109
Tan 16.1 = .289
.289 is greater than .2857 so you should be able to hit any sat with an elevation of 16.1 degrees or higher. Of course you'll gain some height with any pole, so the actual minimum elevation will be lower, but I think you get the idea.