After defeating Lord Nightshade we get the quest 'The Sixth School' from Merle Ambrose. Before we can go into Nightside we have to 'explore the Rainbow Bridge waterfall' then we should be able to go into Nightside. I remember having a problem running through the door and needed to go backwards into the Commons again before retrying, then it allowed me through.
If you've got the quest 'The Sixth School' and the directions are to 'Explore Nightside' but can not go through the door after logging out and back in, going back to the Commons and back to the entrance, then I'd contact support@wizard101.com to see if there is a bug preventing it from opening to you.
When you contact support give them your username (not password), the name of your wizard that's affected by the issue, and a brief description of the problem. You will get an auto-response letting you know they have received your ticket - this is not their actual response. They'll only reply to the email associated with your account so make sure to use the right one or update your account details if it has changed. They will look into your problem asap - sometimes it's super fast but over weekends and holidays it can take a bit longer. It's usually quicker but will be seen to within 48 hours so please be patient if it takes a wee while.
I hope you've already gained access by the time this is printed
