Dear OP
I signed up here just so I could send you some support, because oh my goodness there are a lot of mean people on this site. Seriously, maybe look elsewhere for support, because a lot of the posts saying you're overreacting, seems to be overreacting to your being upset!
I'm not the only one to notice this, but clearly lots of people have missed the bit about his lunchbox being clearly labelled, so that's not the issue. Reading through your posts, and also reading between the lines, it seems to me that this is not about him just having bread for lunch, but rather your anxieties seeming to be realised, and on his very first day.
My (totally unqualified!!!) thoughts are that your upset is coming from lots of things all coming together.
In one of your posts, it seems you had fears that his move up to "bigger" school/classes would mean he wasn't as well cared for as he has been until now, and despite your very best efforts to make his first day at school a happy and positive one, if feels like all your fears were justified. And that was reinforced by your son being upset (I think when you picked him up, and not because you're somehow projecting?), and then you heard the story from him because you kept digging for information, that it didn't come from one of the adults, that it took TWO conversations to get the full story (that's not a consequence of being too busy during the day to do everything needed), and a totally uninterested response from those adults, both towards your son and also you. You were hoping for some reassurance and even a bit of sympathy or understanding about your concerns, and your son's state (physical and mental), but that need wasn't met.
And who knows what else is going on in your life, and your general parenting anxieties!
And that's why it's getting worse as time goes on, and the intersection of all these things gets bigger as you think about, with no outlet valve because you're not able to address the issue with the school at the moment (not until at least the next school day), and then the pretty awful pile-on here making it even worse, although fair to say some posters have been nice and helpful.
Maybe this is all totally off track, but maybe something like this?
I think that there is an issue here that's worth addressing, and potentially the resolution is to talk to someone at the school, but maybe first take a little bit of time and work out just what it is that is making you upset. And then, if you do approach the school, go with a solution that is easy for the school to put in place. It's ok if the solution is simply that the school just explains the process works, maybe an apology that recognises you and your son's feelings, some reassurance that this is genuinely just a momentary snafu, and not some ongoing issue that will continue, or some scary sign that there will be lots of problems down the road! etc.
I do understand the advice to not be THAT parent, but at the same time that doesn't mean that you have to just push down your worries/fears/concerns without them being addressed either. Hence,if you decide you to want to go to the school, go with a clear short message, framed positively, with a potential solution. Although I get the feeling that at the moment part of your distress is being fuelled by not knowing how their processes work, and so also how it went wrong (especially when his lunchbox was clearly his), so might need a few possible ideas and be flexible while listening to them. All of this depends on the culture of the school, too, what they want and expect from parent involvement.
It doesn't sound like you have to do this the very next day, I think the other posters are right that it's most likely they will be watching your son's lunch like a hawk! :-)
waiting for the new school year to settle would be a nice thing to do for the staff, and more likely to give you a good reception if/when you chat with them. And gives you time to think, and if it does happen often in the next days and weeks, then you definitely definitely are entitled to be very upset! ;-)
But overall, no, you're not being unreasonable. The very fact that you made this post with that as the heading, shows you have some insight, you're not just going on some rampage! Maybe it's possible that what you think is the central issue, isn't actually the central issue? But you are entirely entitled to feel upset with the situation, and also to do something (positive/productive) about it, just not while you're upset.
Most of all, be gentle with yourself, you're doing great :-) xo