Oh your poor DS.
Yes that all sounds like extreme anxiety behaviour (and v similar to my DS). Lots of hiding under desks here too (and some throwing too!).
I can't tell you the difference being in a small nurturing setting who understand his needs has made.
I have to say ours has been a long journey - despite DS being diagnosed at 5, it's taken nearly 5 more years to get where he is now.
His first school couldn't get their head around how to support him because he didn't present how they imagined a typical ASD child would (of course there is no such thing anyway!).
They totally saw his behaviour as a choice.
Reward and punishment don't work because he can't choose not to do something when actually what he's feeling is extreme fear and anxiety.
Much better strategies here:
www.pdasociety.org.uk/resources/education-resources
My biggest learning in his all is read and learn all your can - you will generally always know better than school how to support him so don't expect school to be the experts.
Learn the law and what schools should do in terms of the graduated approach to support - but also if you're teetering on exclusion / them 'persuading' you to take him elsewhere then you might want to fast track straight to EHCP assessment.
An assessment will (should) include an EP and OT report to build a full picture of your DS's needs.
It's not diagnosis dependent (however it would be useful for you I think).
The best thing we did in year 2 was pay for a private OT assessment of DS.
For good advice on what the school should be doing you can book a call in with Ipsea or SOSSEN.
You will also have an independent advice service in your area (altho they often just tell you LA policy rather than the law which you will find can differ greatly).
https://councilfordisabledchildren.org.uk/independent-support
Finally if you do apply for an EHCP assessment be prepared to be told something like he won't get one if he's not x years behind - that's not true at all.
Plus the LA will probably say no (ours did). You can appeal the decision and 85% of no decisions are overturned at appeal.
LAs just try and put people off.
The SN chat and children boards are busier than this one and contain many people who have been / are where you are right now. 