Though I'm ignorant of the preceding setting, Yuhana declined the other character’s suggestion for Yuhana to join the production work of snow statues by adding an unnecessary and problematic reasoning – the teachers are letting their pupils engage in heavy work of removing accumulated snow from the schoolhouse because it’s troublesome for teachers to do it.
The other one hanged on this remark of Yuhana, and warned her not to say such an insensible thing – to let school children clear the snow, even she envisaged so.
Yuhana responded “It’s true, exactly” in realization of, and remorse for that her own remark – letting pupils engage in heavy labor of clearing snow as a part of extracurricular activities was thoughtless.
So, Yuhana’s 「全くだ」is not an irony, nor it does contradict with preceding passage. It’s Yuhana’s self-reflection for admitting her reference to letting school children clear snow was inappropriate rational for her not willing to join the snow work as well as a remark to be made by an adult. She admits it's her gaffe.