Do you have a luggage rack on your bike?
Here you see lots of people placing a Steco Monkey on their luggage rack, to carry their school luggage very secure with them, without having the weight on their back all the time. Watertight covers to put over bags are common here too, because with side wind and lots of rain, water will often still creep through watertight school bags. (Through the zip-locks and underneath flaps)
Makes cycling a lot more pleasant, with the weight on the back of the bike, instead of on the back of the cyclist. I never have the weight on my own back, when cycling.
Some people also carry the luggage in front of their bikes, that is OK if it is not too heavy, and if there are not too many differences in height during the trip. And it will make steering less agile.