When you are in the Slytherin common room and need to get out, hide behind the pillar on the right side and wait for the girl to pass. Next, go into the room on your left and stay behind the door. The girl will return and look around. She will not see you. After she walks away, run and jump to avoid being captured. As soon as you exit the room, go to your left. Once there, start running to the door to avoid being captured. This may take a few attempts, as there are many people waiting by the door. Run very fast instead of creeping there -- the Slytherins do not catch on very quickly that you are there.

To escape the Slytherin, you need to first open a painting in the painting room before the common room. It is the second one the left. When avoiding Slytherins, run to the library that you will find and go to the painting (behind). Open it, and it will be much easier because you will be almost at the end.