It comes as the Illinois Education Labor Board looks into claims that CPS is interfering with the vote.
The teachers union filed an unfair labor practice charge against CPS stemming from a letter sent by schools CEO Jean-Claude Brizard to all teachers this week.
The union claims the letter, sent on the day that the strike authorization vote began, was coercive and threatening and designed to derail the process.
School officials say Brizard had every right to communicate with teachers that the vote is premature.
Final results of the teacher vote aren't expected until next week.
