Trump got a medical exemption. That's common to many people rejected for military service. Whether it was true or not is debatable. You do know that about 30 to 40% of all draftees are rejected for military service don't you?
In Clinton's case, he knew he was going to get drafted. He signed up for ROTC in Arkansas to get an exemption. Upon that being granted for entering ROTC, Clinton dropped out of the program thus avoiding the draft. There's no debate there that his actions were done to avoid the draft.