This information comes from an ajarn reader.
It's common for schools to hang on to your teacher's license (both the permanent version and the version issued to that school for your current contract period) while you're working there.
Schools sometimes like to hang on to your passport and your blue work permit book too, officially for safekeeping, unofficially perhaps in an attempt to stop you disappearing at the end of the month.
Remember that your passport is yours, the school has no right to keep it and it should be kept with you.
The work permit has to be kept with you OR at your place of work during working hours: again the school has no automatic right to keep it in the school safe forever.
At the basic minimum, keep a copy of the work permit in case you need it to refer to the number or issue/expiry date.
When you leave your work, the school must give you your permanent teacher's license (but not the current one issued for your employment) whether you leave Thailand or remain to go on to a new job.
Check you have the original license with the original photograph and stamp on it. They have no right to keep the original permanent license and give you only a copy. It's yours, not theirs.