PA is hardly a "diagnosis," maybe a symptom, or more likely just his reaction to a situation in which he does not have a lot of control. This may just be the teachers perception too--maybe she is more demanding or has different expectations of his behavior then at home, and when he does not immediately comply she is attributing it to a PA pattern. As parents we have different perceptions, there may be things we don't see. Talk to others that know your child and that you feel would be honest with you. If no one else is seeing this maybe it's the teacher's issue and not your child's. If others are seeing this behavior, just working with your child to help identify feelings and express them appropriately, or accepting the things he does not have control over in school, may help reduce some of this PA behavior.
A teacher SHOULD NOT be doing any diagnosis other than maybe mentioning something to you and/or the school counselor. She hasn't the training and idiots who do this type of stuff are why we have so many normal kids on meds for ADD.
Odd that she mentioned it in "passing", as in, during a holiday party or a chance meeting in the teacher's lounge? I really think that's very odd, to mention this off hand rather than have a meeting about it or note it on a report card.
What behaviors specifically is she categorizing as "passive aggressive"?
If he's choosing to ignore communication when it interrupts something he wants to be doing instead - that is passive aggressive. The quick definition of PA is "habitual resistance to demands". If he's simply unaware momentarily that she's trying to get his attention, that's inattention, not PA.