In Nuernberg and Frankfurt airports you can leave your luggage at special counters.
It's much cheaper though to rent a locker at the railwaystations (no problem in FRA, because the station(FERNBAHNHOF) is part of the airport.
In NUE the railwaystation is located in between the airport and the stadium, so I'd recommend to drop the luggage there (travel time from the airport to the stadium is 40 minutes by subway, and you have to change trains at the main station anyway...).
Price for the locker should be € 4 per day.
Good luck!