My teen daughter does her own bento in a bento box every morning. It takes five seconds and consists of putting leftovers from last night in a box (assuming we had something like grilled fish or chicken). If the leftovers from last night are not the kind of thing that works as bento, such as a curry, she makes a peanut butter sandwich. And wraps some of last night’s salad in foil and shoves it in.
Fancy bento are not a thing for secondary school, they are as per the above, basically.
Elementary school kids usually have kyushoku (school lunches) which are really, really good! You might have to make bento on certain occasions, like a half day at the end of term, or for school trips obviously. However, when you do do bento, you usually just do simple stuff based on leftovers, as per the above. Actually, most of the public elementary schools round here specifically ban “Kyara-ben” (bento made into fancy shapes) for the occasions when you do make bento, because it would encourage kids to fuss about it and put pressure on parents, and also because there are concerns that all the extra touching and handling of food that goes on when it is cut and shaped is not very hygienic and raises the risk of contamination of food.
Kindergartens for 3-6yos (which traditionally have been aimed at non-working mothers) do have daily bento in most cases, but again, a lot of them ban the fancy shaped “Kyara-Ben.” I know a couple of mothers who send their kids to kindergartens that have daily bento and are really “into” making this kind of fancy stuff, but most are like me and just shove leftovers in a box (or better still, make your kids do it themselves) on the occasions when it does need to be made.
In any case, these days most mothers in Japan are working, so we usually use daycares (which serve school lunches) rather than kindergartens for our 3-6yos, or send them to the kind of kindergarten that is adapted for working parents and serves school lunches instead. So again, very few occasions when you need to make bento.