The very idea of extra credit questions annoys me. By all means make the last question a challenge question but it should still be part of the 100%. If you grade to a curve then surely the top should still be 100% the bottom 0% and the median 50%. More than 100% is just a nonsense. You can have a 200% increase but you can't get 108 out of 100.
ETA. Sorry but it seems like a grade inflation system. You make the basic test fairly easy so a lot of people score really highly then to reward those who actually are doing well you give them a whole extra section to score even higher. Really many of those people doing really well on the basic test should be getting 70% on a proper test and when I was at university 75 to 80% was a very good mark, 90% was amazing and I think the highest I ever heard of was 98% because no one actually is perfect.
In DS's school, which claims to be "rigorous", the extra credit questions, which are challenging (e.g. think along the lines of competition math problem sets) are actually used to weed out the kids who are working at a higher level in the subject as compared to the rest of the class. A math test that is long and has hard problems is difficult to complete and check for errors in the standard 40-minute format and the extra credit question which takes a few minutes to process and solve is meant to challenge the child to think on their feet, solve problems on the fly, etc. Some schools use this as an indicator that the child has attained mastery and is working above grade level which enables them to finish the test in a shorter duration and to solve extra challenging problems in the leftover time.
DS's teacher only gives credit for the extra questions if the child has finished the regular credit questions.
It turns out that not many kids get around to solving them on a regular basis and those who do are considered to be working below their abilities because they could finish the regular test and then tackle a challenging problem in the same time that it took others to complete the original test. A score that is higher than 100% in such tests means that the child is probably working at a higher level on that subject through self-study and is performing at a higher than expected level for that grade and is a candidate for subject acceleration. They ask such kids to write an essay on whether they think that their placement in the subject is correct or not and based on that essay, they meet with the child and decide to test them for subject acceleration.
(kids being chosen for getting > 100% is the case for NT kids whereas there are accommodations in place for kids who need them such as taking the test in the library for longer durations etc.)
