There is another reply earlier on this post that is very helpful on 501C3 tax issues. the indication is that you must file for exempt tax status by the 27th day of the following month that you elect to form a tax exempt organization.

Does anyone know what the grace period is for applying for a 501C3 tax exempt status?

This is with respect for the transition from PTA to PTO and the tax exempt status for your organization as an entity.

Thanks