@aahah
To be honest, I understand the school's concern! Anything that becomes too popular can be detrimental to an extent, and video games are always a sore spot for schools. Their fear is that advertising a video game could result in rowdy/disrespectful behavior.
This gets wordy, but my main argument is that their request is soft and petty. It is not WRITTEN or global rule of the school, and thus does not override your decision as a parent.
- I would first argue that it is not directly impactful or harmful.
It is just a logo. If it depicted killing another character or something more questionable, maybe they would have a point. But to carry a lunchbox with a simple design from the MOST POPULAR THING for your child's demographic is well within reason.
- Secondly, I would argue it is not the school's purview in the first place.
I very strongly believe that it is not the school's responsibility to manage the likes/lifestyle of its children or parents. Nor should it be their priority. It is their responsibility to teach your child essential knowledge and life skills where applicable. Policing style choices, unless
WRITTEN AND ENFORCED, should not be the priority. For example, your school uniforms are rightfully enforced by the school. But unless the dress code specifically has a clause prohibiting the Fortnite logo, it is not their choice. It is their preference. I don't care about their preference, and I don't think you should either.
Schools have agency with three things:
- How our children spend time during the day (time blocks/free time/classes)
- what they learn during their stay (class curriculums)
- to an extent, acceptable interpersonal behavior (written behavioral rules)
A school can take action against a rowdy or disrespectful student, or one that skips his/her classes. These are hard, written rules, ones that both you and your student agreed to upon admission. A school's soft preference does NOT take precedence over your or your child's personal aesthetic/lifestyle choices. This is a slippery slope that I would advise you to consider. If they control your child's lunchbox, what else might fall under their control? What if he brought a religious lunchbox?
Their point is that your child is advertising a property that they find distasteful. But that design is not inherently hateful or violent, and distaste is not a valid reason to question a parent's judgement.
How to react? I would suggest to them that their priorities lie elsewhere, and assure them that you watch out for any unwanted behavior that originates from the lunchbox. I would aim for appeasement, because it sounds like they already have it out for you! 
I absolutely understand them wanting to take preventative measures, but frankly they shouldn't be so bored.