Irina only qualifies for Gifted.
In Irina's case; Irina learned those 10 languages and her ability to play instruments without studying for it. She learned those with little to no time because she needed them for assassination. She herself cleary implies
she learned those languages by picking up what the other said in those countries. I personally don't see someone gifted being able to do this.
In Gakuho's case; his lessons are 20 times faster and 10 times easier to understand, he learned how to beat a black belt at Karate just by watching his moves, brainwashed people, broke others their limiters, drive them to madness, etc., his lessons being 20 times is obvious reference to Koro's mach 20 speed, he was compared to Koro within his arc multiple times not by students alone but by Koro himself too. Not to mention, he does everything I named with pure intellect.
In Nagisa's case; genius in assassination considering, he was constantly called Genius in assassination by top-tier assassins, his classmates, and Koro who was the first the God of Death(for context God of Death is the greatest assassin) should be okay. He also solved half of the exam that had college level material in it(argument for above average or high academically).
In Itona's case; genius in tech, considering
he can make a mini tank that can shoot stuff,
he later added things wanted by other class 3-E students no matter how irrelevant or ridiculous they were, doesn't sound far fetched to me.
In Kaede's case; she was able to pass Kunugigaoka transfer student exam in her first try(iirc she was the only one to pass it, but her profile says otherwise, I don't want to misinform nobody.) took those tentacles that was made by Yanagisawa, injected herself, and was able to control it by herself even tho, you need to do many daily treatments and practice to do that. Itona for example even though he was super visioned by Yanagisawa wasn't good as her was.
In Karma's case; Karma is the one of the smartest if not the smartest student in Kunugigaoka Junior High School(which is a prestigious school). Even while
he was in junior school, he was casually solving a year above material and learning with a short teaching session, he has shown quick decision-making feats such as coming with a plan to beat Itona on the spot even though he had a hostage, outsmarting all of class 3-E, coming up with a way to defeat enemies far stronger than himself like Craig houjou and his men. When it comes to academical abilities, he can get full marks at an exam that had
college level material in it
with relative ease when he takes exams seriously.
Just to add a few supporting evidences to those, these students are all capable of making houses, pudding at the size of a building(They build everything needed to make that pudding stand on it's own.) coming up with plans to sneak into hard guarded places, hijacking a space station(They did get help some help from Koro for this, unknown to what extent.).
Not a fan of whataboutism but there are multiple characters that gets Genius rating because they aced their highschool exams or they know how to gamble, I don't see what is the problem with giving Genius to middle schoolers who can ace exams with college level material in it.
If these don't qualify as Genius, then that is that I guess, accuracy comes before whatever I think. Sorry, it ended up being kind of long.