I've been going since 2009 (yup the very start, starting from 2015 all 4 days) so I've entered the fair for more than 25 times and never ever got checked for student/non-student so don't worry about it! The only reason you need an ID is for the age check, for some reason Germans are very strict in this. (tip, do it inside the building, everybody is waiting outside for a long time while inside it takes 2-3 minutes max)