A lot of the GuP characters (although probably not Fukuda or Katyusha!) could pass for more mature looking with a change in outfits. The "Great Tankery Operation" outfits have a category that are categorized as "occupational", and someone unfamiliar with the characters might not be able to tell they were high school students by how they look in that attire, as well as in some of the "casual" outfits. https://gup.fandom.com/wiki/Great_Tankery_Operation:_Costumes?so=search#Occupation
40 years old!Nishizumi MihoNishizumi-san!?
What's with that figure!?Nishizumi-dono has grown up de arimasu–!?I've noticed something.The customers, they want MILFs, not high school girls!!Customers?