Last two are nice, but the editing really kills it for me. First two, not enough of the subject is shown. Try including the environment that the skaters are in, for instance, with the last two, show the quarter pipe that the guy is hitting, more context is not always good, but in this case the pictures would benefit from it. Also, when shooting anything with straight lines, for example the deck of the pipe, try to keep it relatively flat, otherwise it causes your photos to look lopsided. Overall good job though, focus is pretty good, just get out there and shoot the park more, try new and interesting perspective and just have fun, thats what skate photography is all about.