I had the same problem with 4 teen sims in the same household getting up from the exam to do random things like shovel snow, play violin, order food, etc. 3 of the 4 would cancel the exam interaction all together and then get up to do other tasks while the 4th kept working. one of my sims did this enough times that she didn't get credit for taking the exam at the end of the school day. In the second exam about halfway through I tried turning off autonomy just for the duration of the exam. didn't have any more problems, but there wasn't much longer left in the exam at that point. will update if it keeps working.
EDIT: I had autonomy turned off for the next exam and only one of the four sims got up from the exam one time, as opposed to almost all of them several times. Not a complete fix but definitely an improvement. Only problem is if I forget to turn autonomy back on after school.
Anyone know a mod that helps fix HSY issues like this?
