I have to carry my rucksack around all day and some days it is very heavy. Is this going to do long term damage to my spine?

In general ergonomists are concerned about the loads that are being carried by school pupils and how young people can be expected to move several kilograms of text around the school - certainly one part of the solution I would like to see would be the provision of secure locker space in which the bulk of the books could, when not needed, be left. However, there are some basic rules that you, and those that you know, should be following to minimise the risk of injury:
1. Minimise the weight to be carried - always a good rule!
2. Make sure that the weight is carried symmetrically about the spine - so your rucksack would sound to be good, providing you don't carry it off one shoulder!  (Just look, from behind, at people walking and if they carry a bag on one shoulder expect that shoulder to be raised and the upper spine bent over.  Even small bags or cameras can result in that "un-level" shoulder posture.  Now we do know that a "raised/lowered shoulder" posture can be bad for your health.  It is also, incidentally, a reason why many otherwise well qualified people are not accepted as "catwalk" models"; a job for which a upright, symmetrical posture is required!)
3. Make sure that your bag has wide straps to spread the load and that it sits comfortably on the back - avoid, for example, books packed so that they cause discomfort.
4. Pack your bag so that the heavy (dense) material is at the bottom of the bag, if it is a full sized, correctly fitting rucksack you can transfer some of this load directly to the pelvis and take it off the spine/shoulders.
5. Take particular care when putting on or taking off your bag; not only will the load be asymmetrical but your posture is usually twisted. Indeed, even under common, near ideal, conditions I would not "recommend" adult females to lift 3kg from the floor.