http://www.stopscratchedglass.com/The site is fairly informative about scratching tempered or heat-treated glass, which you would almost definetly have in a sports center, the scratches are probably caused by whats call "fabrication debris", which are microscopic glass particles that become fused to the surface of the glass during the tempering process
The best way to find out if this is the problem is first check for the tempering stamp which should be on one of the bottom corners of the window unless its hidden under the seal, assuming the glass is tempered the scratches should be almost exclusively on one side of the glass, if they are then take a close look at one of the scratches ,follow it to its beginning and if there is a thicker section of scratch( sorry don't know another way to describe it) then its definetly "fab debris"
What happens is your scraper while going over the surface of the glass pops a glass particle out and pushes it ahead of the scraper causing a scratch and the thicker section of the scratch is where the particle came from
Its a very common occurrence here in the US , did not think you lads had many problems with it over there
If all the above is correct and it is fab debris your next problem is getting anyone to believe you, as Nat said " get a waiver signed" I think theres a sample one on that website