What is the Educator Expense deduction?

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.