This is why it's great you did your research BEFORE getting a GSD realizing it's normal for them to bite WAY more than other puppies.
We even made a sticky for it ---> Puppy biting... Hints and tips to help
BTW, the more you can work on ---> Top Training Expectations for Puppies
and continue socializing/attending puppy classes, the better luck you'll have too.
THIS IS NOT AN E-COLLAR ISSUE IN A 15 WEEK OLD PUPPY
This is merely a TRAINING and consistency issue that WE need to learn/teach our puppy. And until we learn our part and TEACH it, then the biting will continue.
At 15 weeks old, how many miles/hours are you able to walk with your puppy each week? --> Proper Exercise for puppies
It's amazing how a tired puppy is a GOOD puppy. So I know when my pup is being a crazy wackjob then clearly I've gotten to wrapped up in my life over the past few days and need to drop everything, pack the pup into the car, and got socialize/wear that puppy out. Off leash is best.
What is your instructor saying in the puppy classes? I know my weekly classes with my 13 week old pup are helping me tremendously.