From the IRS:
If you are an eligible educator, you may be able to deduct up to $250 of expenses you paid for purchases of books and classroom supplies. These out-of-pocket expenses may lower your 2007 tax bill even if you don’t itemize your deductions.
The deduction is available if you are an eligible educator in a public or private elementary or secondary school. To be eligible, you must work at least 900 hours during a school year as a kindergarten through grade 12 teacher, instructor, counselor, principal or aide.
Qualifying Expenses: You may subtract up to $250 of qualified expenses when figuring your adjusted gross income. Qualified expenses are unreimbursed expenses you paid or incurred for books, supplies, equipment (including computer equipment, software and services) and other materials that you use in the classroom. Supply expenses for courses in health and physical education are qualified only if they are related to athletics.
To be deductible, the qualified expenses must be more than the savings bond interest excluded on Form 8815, any nontaxable distribution from a qualified tuition program, and any tax-free withdrawals from your Coverdell Education savings account.
The deduction for educator expenses can only be claimed on Form 1040, line 23. For more information about this topic, see IRS Publication 529, Miscellaneous Deductions.