Have you checked google earth for any elevation change between your take off point and the school? You can often have a slight incline that you don't even notice end up being a change of 50-75 feet. Also, your altitude readings are subject to some error, sometimes 30-50 feet or more. Its easy to see that if these all add up to your detriment..well, you can get a situation like you just ran into. I like to all an extra 100 feet or so on a first flight in an unknown area, look at the video, and then decide if I can fly it again a bit lower. Good luck on your future flights! You did get lucky this time, but I'm sure you learned from it. I'm sure there are lots of other folks that are not so lucky!