I'm in the minority who think it's an OK idea and don't understand the mockery the OP is receiving.
I interpreted it to mean that DS might need to use first aid on someone else, not that he would necessarily use it on himself.
As for some of the comments:
There don't need to be lots of stabbings in your local area to make you concerned about knife crime. It might only happen once but you could be the bystander who could make a difference between life and death.
Collect him from school so that he's not at risk: so he has to be accompanied by an adult whenever he's outside the house, until he's at adult? Don't be so bloody ridiculous.
Move to a different school: it's not the school that's the problem, there's the possibility of knife crime in the area. And no, that doesn't mean the OP is necessarily living in a gang-infested high-crime corner of London. I live in a safe part of London and there have been very occasional street stabbings in the wider area over the past 10+ years. And one lad who was stabbed while visiting a different part of London.
It's unlikely to happen, so what's the point: the likelihood may be low, but the possible outcome is extreme (ie death). Weighing that up, it's worth making an effort because the value of your action in the, admittedly unlikely, situation might make the difference between someone dying on the pavement or surviving to live many more decades. Or maybe only surviving a few hours, but that's enough time to get to them hospital and having their loved ones joining them.
Stabbing scenarios or not, learning first aid is a good idea - for everyone, including the OP her/himself. I'd also recommend the CitizenAid app which gives advice for a range of emergency scenarios (eg multiple casualties in a major incident, terrorist attacks etc). Obviously read it occasionally first, don't wait until you're faced with an emergency. And, yes, you'll probably be too panicked to scroll through your mobile phone for advice, but it might just give you confidence to deal with the situation or a memory of what information to give to the emergency services.
www.citizenaid.org/