It would seem like it should be obvious that time and a half means 50% more time on everything, but I have heard of some strange applications in other districts. In your case, I would really pursue a clear set of rules in writing. I have heard of refusal to add extra time or to otherwise discount the time/speed factor in IQ and/or ability type tests in cases of testing for program entrance. This is justified as inappropriate distortion of the testing instrument. However, the district apparently gets around allegations of discrimination by allowing a lower entrance criteria instead. So for example, they won't accept a 130 GAI, but they will adjust and permit a 115 FSIQ for documented conditions; similarly, they may just require lower scores on the SCAT. In your DS' case, it seem that the district has chosen a screening test and a final test that strongly emphasize speed. (I know that you don't need to actually do most of the calculations on SCAT to figure out whether it is <, > or =, but speed is still a big issue per my kids). Access to accelerated math shouldn't be so dependent on speed in the first place. Good luck.
