I'm going to speculate a little, in a moment, but first our own anecdote:
Our #1 has been having a similar situation. Also net grade advanced two years, and an additional year in math. Also seemed to bump up against something at precalculus. We homeschool, so rather than grades dropping, the pace of math progress just slowed to a crawl suddenly. A similar phenomenon with conceptual understanding, but many minor (careless) computational errors. None of the medical or friend issues you have in your situation, and nothing special happening elsewhere in life, that appears to be having deleterious effects. Our DC does have some ADHD/executive dysfunction-esque characteristics, but is neither diagnosed nor treated.
Here are some of my thoughts: I wonder if there's a leap in maturity that is necessary when moving into the increasing abstraction of precalculus, much of which is harder to connect to meaningful applications (especially the way it's taught in North America). In addition, the number and complexity of computational steps in each problem demands greater executive functions to keep organized, sustain concentration, and self-monitor (screen/check for minor errors) all the way to the end. The difference in frontal lobe development between an early adolescent and a late adolescent is quite substantial, and may be enough to account for the experience of these early adolescents with precalculus.
You mention messing up calculations toward the end of problems. That sounds consistent with EF difficulties, as that is the kind of error that happens when one is unable to sustain attention throughout the entire problem/exam. (Not saying this is your child, but for example,) Even if attention is managed with psychostimulants, the underlying disorganization does not necessarily improve much. And self-monitoring skills are both developmental and learned. If they have not previously been necessary, and one's native ability for self-monitoring is marginal, then they will not suddenly appear without instruction and practice.
ETA: Oh, and I've noticed that having an adult scaffold EF helps our own child substantially. Scaffolding consisting often only of company, "yes, you can do it", and "please check your work". And the occasional, "hyperventilating will not help your math brain work better."
Last edited by aeh; 11/17/15 08:28 PM.
