it is part of their job to look at whether he is ready to try some pt ms if this is the norm for the school, so they are within their remit to raise it as a discussion point at school. and they have obviously discussed it with you in the past (and you have agreed).
you just need to reassure him that nothing will be happening, it was just a conversation. you can discuss with school that their method of raising the subject caused him to be anxious at home (they need to know this anyway) but tbh i doubt that an nqt with no experience in sn would get a job in an asd unit. and if he is happy, she can't be that awful.
getting yourself in such a bother will be having a detrimental effect on ds. you need to be calm and reassure him that nothing is happening. if he's been out of ms for a year and you are anticiapting that ds will lead a reasonably independent life in the long term, it's reasonable that they look as to whether he is ready to try some pt stuff accompanied by a TA. they won't know if he's ready unless they talk about it, both to him, and to you.
i suspect that your own anxiety about how he was before is colouring both of your reactions to this possibility. he is happy and settled now. if everyone is calm and suggests to ds that he could (if he wanted to) try it once or twice to see, with no obligations, it might be easier for everyone concerned than discussions of cords around the neck and drownings in the bath. the 'visits' might be no more than a walk around the corridors at first. noone is going to be bullying him.
and walking around the corridors when lessons are in progress and he is with a familiar TA would be a great way to start desensitising ds to the setting.
sometimes as parents we have to take a deep breath and look at the bigger picture. try and look at it less emotionally and try and work out whether this is something that you want ds to be able to do in a safe environment. you still have the right to believe that he isn't ready. but try and do it dispassionately. and discuss it with school. school are obviously trying to see if he is ready. that's their job. you can have input, but they are more likely to listen if you can put your point forward as it stands at the present, rather than listing issues that are not current. ds is happy and settled, but may find it difficult to move to this next step. he may find it okay with good support from the school. he might even enjoy it. he might not. school might decide he isn't ready. but they will be listening to both you and him before they decide.