As a junior you should be one if not THE last one to leave. This is the time to prove yourself to your colleagues and your boss. If your hours are supposed to end at 5:30pm stay at least until 6:15pm. Spend the extra 45 minutes looking up cool new trends or learning some new way to utilize social media.
If you leave on time and the rest of the creative department is working on a presentation ASK if there is anything you can help with. You leaving before everyone looks poorly on you.
You are a junior, we know you will be slower and need help. That's why you are a junior. What we are looking for is that fire. That desire to be there and change society with a great idea. If you don't have that . . . you need to find a different job.