From my own experience, before applying for work permit, one must have a Non-B visa. But if you already have a non-B, I don’t see why you need to leave in order to apply for the work permit.
For my BOI company, I originally applied as vice-GM, got rejected and told to amend to “Director”. Then got rejected again and told to amend to “Managing Director”. Seems they will only allow once executive level work permit for the company.
i was told I only need to bring the letter from school but immigration said I needed to make copies of my passport with the visa and all the thai stamps related to this visa. Have to come back again basically. The officer told me no problem even if I cancel since it expired, I won’t get fined.