The IRS also applies its "ordinary and necessary" rule here. To be considered ordinary, an item purchased for your classroom must be something that is common and accepted in the education profession. A necessary expense is one that is helpful and appropriate, but it doesn't have to be required to be considered necessary.

So buying a videotaped production of "Death of a Salesman" to help drive home Arthur Miller's points to your students would likely meet tax muster. But purchasing a new HD television upon which to watch it instead of using your school's working-but-old set may raise some IRS eyebrows.

Circumstances could limit expenses
In addition to the eligibility requirements, the IRS also has set some limits on the money spent on school supplies. Most notable is the restriction that could affect teachers who are continuing their own educations.

The IRS says when an educator uses any tax-favored funds to pay for his or her own schooling, those amounts must be subtracted from the total the teacher claims under the educator expenses deduction.

Take for example Joe Jones, a high school English teacher who is working toward his master's degree in literature during school breaks. He cashed in savings bonds to pay his tuition and excluded the bonds' $150 interest from tax. He also spent $200 for books on Shakespeare to distribute to his 11th-grade students. He must subtract the $150 in tax-free interest from the $200 for the books, leaving him only $50 to claim under the educator expenses deduction.

The same rule applies to nontaxable earnings a teacher gets from qualified state tuition programs or tax-free withdrawals from a Coverdell education savings account.

Coordinating classroom claims
The educator expenses deduction is definitely great for taxpayers who don't itemize. But claiming it won't prevent you from taking other eligible itemized deductions on Schedule A.

If you're a teacher who usually itemizes to save on taxes, such as deducting home mortgage interest and property taxes, keep doing that. In fact, if you spent more than $250 on your class, take another look at the miscellaneous deductions line on Schedule A to see if you can use the excess to also claim this itemized tax break.

Just remember to take the educator expenses deduction on your 1040 form, too. For example, if you spend $500 on classroom supplies, claim half of it directly on your 1040 as an educator expenses deduction. Then add the other half to your potential miscellaneous itemized deduction amount.

It might be just enough to allow you to take that tax break, too.

Freelance writer Kay Bell writes Bankrate's tax stories from her Austin, Texas, home. She also writes two tax blogs, Eye on the IRS for Bankrate and Don't Mess With Taxes, and is the author of the book "The Truth About Paying Fewer Taxes."