Better ways to thin the herd
The first two I thought of they did mention in the film that other countries are doing. Killing the oldest 5% or limiting children seem like much more fair ideas.
What's the point of killing a 5 year that fails an academic test rather than a 95 year that's probably suffering from dementia? At least the dumb 5 year has the possibility of becoming an asset to society. The oldest people will never contribute again.
They could also kill all the sick people. Those with incurable though not necessarily fatal diseases. Get rid of everyone with diabetes, Parkinson's, all the quadriplegics etc. Free up the health care system.
Another option is take out all the criminals. Rapists, murderers, thieves, drug dealers. Why should they get to live while an innocent child is killed because of a bad grade?
Forced sterilization is probably the most humane method though. It's how we keep the cat and dog populations down. They could even keep the test and just sterilize the dumbest children so they don't reproduce instead of killing them.