Any amount of exposure you get to German is going to help, I think. I went on exchange to Germany last year. Like your trip though, it was only for a month, but when I got back I felt at least six months ahead of the curriculum.
Try to speak as much German as possible and don't be afraid of making mistakes - they are inevitable, they are forgivable, and you'll learn way more than the perfectionists who choose not to speak up. ( this was definitely me, at times. ) Don't expect yourself to understand everything you hear, but try to listen carefully. Also, demand that the Germans actually speak
German to you; they will want to practise their English and it's so easy to fall into complacency since it's your mother tongue, but don't let them! I actually found that the Germans were really surprised when I attempted my less-than-fluent Deutsch - they appreciate the effort you make, since they're so used to speaking English to foreigners .
In the meantime - and this is what I'm currently doing - expose yourself to as much German as possible! Read magazine articles and books, watch Inspector Rex episodes without subtitles, follow blogs in German. When I was in Marburg I bought a book of Brother's Grimm fairytales; at the moment I'm slowly working through those.
In any case, it sounds like you're really passionate about German and that's what brings real success in anything you do in life. In reality, your study score is just a cold, feeling-less value established for societal convenience; it doesn't always account for the raging, intrepid enthusiasm some of us feel for school subjects....
Hopefully this kind of attitude will be reflected in your mark, but if it doesn't work out that way, well, I like to think that if your heart is set on something nothing can really inhibit you from pursuing your dreams. In short, I wouldn't worry about study scores. Focus on enjoying learning and achieving the best you can.
I really hope you have a great trip and that you learn not only a lot of German but about culture and the world in general!
Oh by the way don't forget to go to parties. The Germans love to party!