Matt cites a reference from the Victorian government stating that a school bag should not be heavier than 10% of the body weight of the child who carries it . So the next thing we need to do is to calculate what a student can carry without being at risk of spinal injuries.
Firstly we need to determine what children weigh, if we restrict this to the 14+ age range (the older children have more need of computers) then children are almost as heavy as they will be when they are 18 and can carry significantly heavier bags than those in the 10+ age range. Also it seems reasonable to consider the 25th percentile weight (the weight which is exceeded by 75% of children). Of course this means that 25% would be carrying overly heavy bags but it does give us a bit more weight allowance.
The 25th percentile weight for white girls in the US is 48Kg . The 25th percentile weight for white boys in the US is also 48Kg . When considering the Australian situation it seems that white children in the US will be most similar (in terms of genetics and diet).
The 25th percentile weights at age 18 are 53Kg for girls and 64Kg for boys.
So the acceptable bag weights would be 4.8Kg for 14yos, 5.3Kg for 18yo girls, and 6.4Kg for 18yo boys.
The next step is to determine the weight carried to school. The weight of one of my laptop bags is almost 1Kg, I think that a school-bag would have a similar weight.
When I was at school I recall that the worst days for carrying baggage were when I had PE (Physical Education – sports), the weight of PE gear was a significant portion of the overall weight I carried. I tried to estimate this by weighing a track-suit top, a t-shirt, and a pair of board-shorts (the only shorts I could find at short notice), and it was almost 1Kg. While the board-shorts might weigh more than PE uniform shorts I didn’t include the weight of track-suit pants. Assuming that a female PE uniform has the same weight as a male uniform is the least of my assumptions.
The weight of a good pair of sneakers (suitable for playing basketball and other school sports) that is in my size is just over 1Kg.
To get a rough estimate of the weight of lunch I put six slices of bread and a small apple in a lunch box and found that it weighed 500g. A real lunch would probably include four slices of bread but the other things would weigh at least as much as the extra two slices. If a drink was included then it could be more than 500g.
So the total is 3.5Kg for bare essentials (including PE gear) without including any books!
It seems that it would be impossible to stick to the government recommendations for school bag weight if a full set of PE gear is included. Probably the best thing that could be done would be to make a school uniform allow wearing sneakers which removes 1Kg from the overall bag weight.
So a bag with lunch and PE gear (minus sneakers) is about 2.5Kg, leaving 2.3Kg for books etc at age 14. As text books for 14yo children are considerably thinner than those for 18yo children, it seems that this might be achievable. Fitting a couple of text books and an EeePC into the 2.3Kg weight budget should be achievable. But fitting a full size laptop (which seems to start at about 1.8Kg) and some text books into a 2.3Kg budget will be a challenge – it might be possible to do that but wouldn’t leave any budget for the random junk that children tend to carry around.
For an 18yo girl, the weight budget (after the basics have been deducted) is 2.8Kg, it seems likely that on occasion the weight of year-12 text books will exceed that. Therefore it seems that the only way of avoiding spinal injuries in year-12 girls would be to have text books stored in digital form on a light laptop such as an EeePC. Rather than avoiding the use of laptops because of weight (as some people suggest), laptops with electronic text books should be used to replace traditional text books! An EeePC weighing less than 1Kg will give a significant amount of extra capacity for any other things that need to be carried. If there is little other stuff to be carried then 75% of 18yo girls should be able to carry a full size laptop plus PE gear without risk of back injuries. If digital text books are used then if in any journey two text books (which according to my previous measurements can be as much as 1.6Kg ) can be replaced with an EeePC then overall something like 600g is saved (depending on the configuration of the EeePC, if one battery was stored at home and another at school then it could save more than that).
It seems that a year 12 girl who has PE and three science subjects scheduled on the same day would be most likely to exceed the recommended weight in the current situation (even without having to carry a spare pair of shoes), and that carrying a laptop with digital text books would be the only way of avoiding back injury.
For an 18yo boy the weight budget is 3.9Kg after the basics have been deducted. So if they don’t carry other random stuff in their bag (I probably had 1Kg of random junk in my bag on occasion) then they could carry PE gear, a full sized laptop, AND a full set of text books.
It seems to me that there is no situation where children would be unable to have a laptop within the reasonable weight allowance if digital text books were used. The only way that the weight of a laptop could be a problem would be if it was carried in addition to all the text books.
One final point, it would be good if books from the Gutenberg project were used for studying English literature, that’s one easy way of reducing weight (and cost). Also it would be good if there was an option for a non-literature based English subject. Knowledge of English literature is of no value to most students. It would be better to teach students how to write while using topics that interest them. Maybe have blogging as an English project. ;)
-  http://matt.bottrell.com.au/archives/155-Laptop-clarifications.html
-  http://www.betterhealth.vic.gov.au/bhcv2/bhcarticles.nsf/pages/Back_pain_schoolbags?open
-  http://www.halls.md/on/girls-weight-w.htm
-  http://www.halls.md/chart/boys-weight-w.htm
-  http://etbe.coker.com.au/2008/02/18/laptop-vs-book-weight/