It’s probably better to take a lighter backpack as it will not have a bad influence on your back especially while you are running or riding a bike. I have found some good options at https://10luggage.com/best-backpacks-for-law-school/ such as Tocode Fashion Laptop. But from my experience, it is better to train without backpacks. As it can be hard to find the backpack that will be suitable for your posture and form of your back.