If your teacher didn't know about the autism, it's entirely likely that she believed DS to be a regular 3 year old having a tantrum.
I think it was very clear that this person did NOT think the boy was a regular 3yo having a tantrum. I think she recognized the unusual nature of the boy's behavior and asserted that the unusual situation the OP was in was NOT of her making. And that's the reason she said something--her very word choice (though awkward) indicated that.
The OP was offended, and I can totally understand why.
But the lady did not offer an insult. She offered unwanted advice, but not an insult.
And I think she clearly understood that the OP's son had a unique set of problems that were not at all the fault of the OP, and also not something the OP was really equipped to handle.
I don't think she was judging the OP's parenting *at all*--hence the term "in over your head" and "I've never seen a child like that." She was recognizing the extremeness of the OP's situation, of her son's disability, and was concluding that no fault lay with the OP at all--"in over your head."
I think it was a stressful time for that lady as well--with all that meltdown and chaos and yelling happening right there.
I suggest flushing that afternoon from your mind unless you have future problems with her.
I agree. I hope you can, OP. And I hope the diagnosis opens doors to resources instead of making you feel "sentenced."