As a school teacher for a charter school am I able to write off more than the given 300 dollars?

No.   If you are a teacher in grades K-12 you can use the $300 educator deduction.   Teaching in a charter school does not change the deduction you are able to use.

W-2 employees cannot deduct job-related expenses on a federal tax return.

Sorry---W-2 employees cannot deduct job-related expenses on a federal return.  Job-related expenses were eliminated as a federal deduction for W-2 employees by the tax laws that changed for 2018 and beyond.  Your state tax laws might be different in AL, AR, CA, HI, MN, NY or PA.

https://ttlc.intuit.com/questions/4482873-which-federal-tax-deductions-have-been-suspended-by-tax-reform

No, the deduction is 300 per teacher. 

The exception would be a Self-employed Teacher who files Schedule C. 

Educator Expense Deduction Link