I also hate how over the top some school have gone with these things (thankfully DDs hasn't), but it is what it is. While we as adults might think just asking someone would be nice enough, it's hard when you are a teenager and everyone else is doing it. What have other kids at the school done? Do they do things that are really crazy over the top, or do they do smaller things like just decorating lockers, making a poster, etc.?
And, umm, maybe it's just my mind, but I'm thinking a blanket might not be the best thing to use as a prop in asking someone to a dance. My mind just goes to how some people might read into it desire for a certain after-dance activity that some kids might want to partake in. And teenagers mind's tend to go straight to this sort of thing - I can say what I think is the most innocent thing and my daughter will read something dirty into it! Not that I think your son meant it in any way like that - it's just that someone who was bent on seeing it that way could. I'm probably just overthinking it but I thought I'd put it out there for you to at least consider.