In a violent, split second chaotic time frame, can that teacher positively differentiate between a "gas mask" and a "face shield?"
Eyewitness accounts during such events are notoriously inaccurate.
Anybody watching TV shows with swat teams, or even local news knows what body armor looks like, however whats different here, is that they rarely wear face shields, and she noted that this person was wearing it. Teachers are very good at noticing things. They inspect and evaluate students every day IMO.