Can I deduct supplies I purchase for my classroom

There is a $300 Teacher Expense credit you can deduct on your federal tax return.    It is under the Employment Expenses category in the Deductions and Credits section.  

If you're a teacher, instructor, counselor, aide, or principal that works at least 900 hours in a school that provides K–12 education, you can deduct up to $300 in unreimbursed expenses for books, supplies, equipment used in the classroom, and curriculum-related professional development courses.

Teacher union dues don't qualify.

Your qualified expenses are reduced by any reimbursements you received from your employer that aren't included in box 1 of your Form W-2.

What is the Educator Expense deduction?