This one didn't occur to me until nearly 20 years after the initial release of the film. When the gang gets to lunchtime, Bender immediately goes around asking people what they've got for lunch and ridiculing the stuff they've got. He then moves off to sit somewhere else. He doesn't eat lunch. On reflection, given he's basically from an abusive household, of course they wouldn't have packed his lunch for him. That's why he fronts the other about their lunches — he's trying to distract them from the fact he hasn't got any.
Why couldn't Bender pack his own lunch?
He probably didn't have any change.
It pretty much matches up with the concept that bullies are often the ones crying out for attention. Bender is a complete jerk, but if you look at it closely enough, he deliberately pushes discussions hard enough that the other kids eventually have to react - like the point where he eventually blows up after showing the cigarette burns his father gave him. Vernon doesn't notice because he's the authoritarian, and had probably dealt with hundreds of people like Bender, but the other kids do. On the one hand, Bender doesn't want to talk about his issues - which is why he deflects stuff. On the other, he probably does want to let it out. It doesn't excuse the way he treats pretty much everyone in the film except Allison, but it's a likely explanation. Even his somewhat triumphant pose at the end could be down to the fact that he just might have some people there for him, now - as well as being just a little iconic.
The ironic echo used in the movie "You don't even count. You could disappear forever, and it wouldn't make any difference. You might as well not even exist at this school." First in the beginning of the movie Andy says it to Bender. If you watch Judd Nelson's face, you can see some hurt that, now that I think about it, is very much intentional. So Bender fires something back and they all move on. Next, near the end of the movie when Bender sarcastically makes fun of Claire's lipstick-talent-thingy. Andy scolds Bender for saying such mean things. Then Bender repeats what Andy had said to him, first to point out how hypocritical he is for even saying that, but then you realize "it really did hurt Bender, what Andy said. It doesn't matter how tough you may or may not be, words tend to stick and hurt and last. Almost Freudian excuse like, but it brings it back to Bender always crying out for attention. He wants people to notice if he is there or not, if he is okay; it's human nature. And the only way he knows how is to act out and be a total jerks!"
Consider what the movie says about the prestigious/successful: Claire's parents are rich, but they're also divorcing and emotionally abusing their daughter, who is unhappy and upset because of the position her peers force her into. The principal evidently doesn't like his job, despite the fact that principal is quite a respected position. Carl, who was once his class's Man of the Year, is a janitor. Brian's parents force him to work hard, not caring about how much stress he's under, and Andy's father has alienated his son to the point that Andy wants to give up a sport he's good at so his father will leave him alone. From this, the underlying message of the film might well be construed as 'Success/riches/prestige don't equal happiness, so don't force yourself or others to achieve them'.
On the topic of Carl, the movie is about breaking down social barriers and features characters that are all ranked differently on the school hierarchy. Andy is at the top (as a representative of the school), Claire is a step below, Bender is in the middle, Brian is on the lower end and Alison is at the bottom (being ignored by everyone). Carl is there to show that you shouldn't waste time worrying about who society says you should care about and just be friends with who you want. This also extends to family, as the five characters have dreadful parents yet are expected to maintain close relationships with them because of blood ties.
The whole subplot with Brian's parents. What exactly is their son going to get out of their extreme pressure to make perfect grades? They could have found other ways of encouraging him instead of mental cruelty.
They probably saw him as a slacker who needed that pressure to make anything out of himself.