Where I have failed on diets before is that I've gone "Ah cr**, I've ruined it all with one meal/night/weekend off. May as well give up".
We all do it. We all have normal lives to maintain too. We all have people in our lives who we have to spend time having fun with, and who don't want to live constantly under the pressure of our diet.
One weekend off won't kill you. Giving up will (metaphorically speaking obviously!). Did you have fun? I would argue that that is more important in the long term.
For today, pick yourself up, dust off the Alpen lights and get back on the wagon. DO go to class, cos that's a slippery slope too. Just know that one weekend can't ruin all your success. You'll be fine!