@LordSugarWillSeeYouNow
I have been a secondary school teacher for over a decade and teach in quite a 'nice' school with lots of quite affluent families. Having said that, it is not uncommon for students to arrive at school without the correct uniform/shoes, purely for practical reasons. As a teacher, if I noticed a student had the wrong shoes or trousers on, I would ask them as they came through the door about it and if they showed me a note from mum (either a paper note or a note written in their homework diary) then I would leave it at that. I don't know what your son's school is like but speaking from myself and my colleagues, we are all human and know that students (boys in particular) go through shoes and uniform like sh*t through a goose. A lot of parents work full-time and I simply wouldn't expect a student's mum or dad to logistically be able to go out and repurchase a pair of shoes that developed a hole after a rough game of football or a pair of trousers or jumper that got lost after PE, left on the bus etc immediately. I have had lots of notes from parents in my time that have said something like "Sorry, the sole fell off of one of Jonny's school shoes yesterday but I will make sure we go out at the weekend to replace the shoes. He will have to wear his trainers until then so apologies." I wouldn't bat an eye, to be honest, and I certainly wouldn't judge them for not having been able to drop everything and get straight to the shopping mall to replace.
I am a mum, am able-bodied and in a good financial position. That said, if my son came home tomorrow from nursery and we found that he had lost his willies or his shoes had broken or something similar, I couldn't logistically go out that very evening and replace it. In all honesty, it would probably have to wait for the weekend or at best, a few days for an online order to arrive. Kids grow like weeds, lose stuff, are disorganised. Sh*t happens, the nursery staff would understand if I said that he wasn't 'fully equipped' that day because something had been lost or torn. They would make do and within a few days I would have been able to go out and replace the item.
In terms of moving forward, can I suggest that you drop an email to your son's tutor or head of year to say that you would just like to flag up that as your son is so tall, and he seems to go through uniform so quickly at the moment, you are limited as to the places you can buy him appropriate uniform. Coupled with that you have a disability which can also make getting to the shops more problematic so you would appreciate staff not making a big deal out of the odd issue with uniform here and there but, of course, you will provide him with a note to explain any issues with uniform as and when they are necessary. It would take a pretty heartless teacher to make an issue out of a chronically-ill mother not dropping everything to run to the shops immediately. They will appreciate you flagging it up in advance and will see that it is not an issue with you being uncaring or thoughtless, just pragmatic about the fact there are sometimes going to be uniform issues which you will always do your best to rectify as soon as is humanly possible.
If you have funds, either now or in the future, would it be possible for you to buy some back-up uniform that you set aside (maybe in the next size up) so that you are not always on the back foot when he grows out of things or has failed to mention his other trousers ripped etc? I don't know how niche the sizing is but I'd recommend keeping an eye out on eBay and snapping any that come up in larger sizes for the future.
In terms of the nasty comments from other classmates, I'm afraid I have no advice there other than to, again, flag that up with the tutor as being something they might want to keep an eye on...
Best of luck! Stop feeling bad about yourself because of trousers. You're doing the best you can and I'm sure once his growth settles down, it will get easier.