Intro and query

Hello everyone

I am brand new ( as of about three minutes ago) to this forum. To briefly introduce myself , I am the mother of three amazing children - my oldest boy is 14 and is fairly newly diagnosed with Tourette's syndrome , we are currently on the autism diagnosis pathway ( where I am almost certain he will be diagnosed  our next appointment in two weeks) , he also has elements of OCD and ADD but not 'severe' enough to warrant a further diagnosis.  He also has rheumatology issues , asthma and eczema and has some allergies. 

My second son is six years old, he has Asperger's syndrome ( or HF ASD as most are diagnosed with now under the one umbrella) he also has severe anxiety and sleep disorder. He is hypermobile , has flat feet and hyperlexia . 

My my daughter is just turned four . She does not have autism ( although she certainly has some sensory issues but then don't most four year olds ? !)  

In a nutshell - My issue is with school . My six year old son attends a strict catholic  achool with a policy of no hair past the collar. my son has serious issues with hair cutting and as such we only cut it as little as possible. ( we have tried everything , he actually also likes his hair long and so it is not only the physical act of hair cutting but also the loss of the hair itself) . 
The achool are aware of his extreme anxiety and autism.  The school and senco are awful with SEN - it is not the best place in that respect .  the headteacher has approached me and asked me to cut his hair. I have e planned his needs before but clearly they have not understood . I explained he has the same person to cut it since being born and she has a torn shoulder and as such cannot do it at the moment- its bad enough when she does it but the suggestion of anyone else to him would just push him over the edge. 
The head then basically asked me why I couldn't just take him to the barbers and hold him? I realised that because he is so conformist and hides his anxiety that they have absolutely no ideA at all. 
So , does anyone know where I stand ?  I know my son is classed as a 'disabled student' and that schools should make reasonable adjustments for disabled children. I have no idea what would define reasonable adjustment ?  shold  I suggest they overlook the hair as a reasonable adjustment based on his sensory needs ( I am certain my paediatrician would support me) . 
my son has no additional help or support whatsoever , we ask for nothing at all. He is incredibly bright and too of his class and is mostly bored out of his mind. The worse issue they experience with him is he can go off into his own little world ( usually when covering things he knows) . 
Does anyone know how i ahould deal with this ? I do not mind him having longer hair at all, his father also does not and does not see why this is achool policy anyway. 
Tying his hair up is not an option at all. He would be mortified and it would break him. 
Thank you in advance for reading my message and for your support and advice . I am just so exhausted working full time and trying to juggle the three children at such varied ages and with different needs - i just don't want any additional worry for my little boy and I know they have approached him directly before and told him to tie his hair up ( for which I did get an apology because it caused weeks of breakdowns and worry , refusing school and crying until he was physically sick) . 
( just for completeness my six year old is on melatonin at night but due to the Christmas upheaval is the worse he has ever been and is sleeping maybe three hours before having. I guy terrors or nightmares or panic attacks and so on) .