As a teacher, can I claim the $250 school expenses if I teach overseas? I am still buying supplies, but the school is not located in the USA

It depends. If you are 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 $250 in unreimbursed expenses for books, supplies, and equipment used in the classroom. All American teachers are eligible for the Educator Expenses Deduction, regardless of where they live. 

Qualified expenses are amounts you paid or incurred for participation in professional development courses, books, supplies, computer equipment (including related software and services), other equipment, and supplementary materials that you use in the classroom. For courses in health or physical education, the expenses for supplies must be for athletic supplies. Qualified expenses also include the amounts for personal protective equipment, disinfectant, and other supplies used for the prevention of the spread of coronavirus.