The difference has diluted severely over the past years. Advanced [i]used[/i] to be an elitist clique of very capable and very prolific writers, but it's essentially become an extension of Casual now. The term High Casual first came into use around 2010 or 2011 when people wanted to bridge the gap between Casual and Advanced; now High Casual and "Low Advanced" are basically the same thing. Your RP would probably fit into both subforums. 

I'm not a fan of that status quo and the first step is to convince people to move their High Casual roleplays back to Casual in the future.