WASHINGTON — For Carrie Uffelman Brake, planning for next school year begins before the current one ends.

The shopping starts as early as April, when she gets the list of students who will be in her third grade classroom in rural Tennessee the following fall. If boys outnumber girls, she will need extra toys to keep hyperactive hands busy. If it is a group of struggling readers, she will need double the number of books.

Bargain-hunting season comes in early summer, when nobody is shopping for school supplies. That gives way to the blowout sales in late summer, when everybody is. Then there’s tax time, and a $250 tax deduction to offset some of that out-of-pocket spending.

For 15 years, the nation’s tax code has recognized teachers like Ms. Uffelman Brake with a small perk that has now come to illustrate a larger philosophical divide as Congress tries to push through a sweeping tax overhaul.