A much-abused study showed that in poor neighborhoods with low academic opportunities, better scholastic performance was correlated with church attendance. This slim thread has been seized upon by religious apologists to justify claims that church attendance improves kids' grades, and is usually accompanied by anecdotes about good church-going children being studious and diligent. The evidence is poor and getting worse, how low can it sink?
You know where it can go, to the worthless world of the internet poll.
Do you think attending church can improve kids' grades?
Yes 52%
No 36%
Don't know 12%
I think it is completely unsurprising that academic scores can be improved with discipline, and that part of that family discipline will be expressed in church attendance, in religious families. It's a long reach to claiming a causal relationship between church and grades, though.