So.. I have some good and some bad news...
good news: I figured out how to change outfits for characters through cheat engine. Yay.
bad news:
1. every class of outfits is handles in a different value. so I'm pretty sure I can't set a shinovi outfit for the school outfit.
2. I need to find the school outift and shinovi outfit memory adress for each character specifically to do the swapping in cheat engine.
so basically it's pointless... given that you can do this for every character through the dressing room anyway.
also switching the value in the changining room and then leaving the changing room made the game crash. doing it in the level select and then starting the level worked.
so yeah the only way I can think of now to get a shinovi costume over a school uniform would be to swap the models in the files...
Edit: I tried using an invalid number to see what would happen. (I saw the list of school uniforms went up to 79) so I entered 80... instantly crashed the game
double edit:
took a stab at model swapping.
well... It worked and it didn't?
it worked because it's the right model... it didn't because well... textures:
I need to fix which textures it loads. i also figured out why each costume has multiple .cat files.
for the example here I will use Asuka's school uniform:
"std00_mdl_hi"
STD = costume type school uniform
00 = costume number
ml = model
hi = high quality model version
then there's for the exact same costume there's this one:
"std00_mdl_lod"
it is the same costume, the only difference is lod instead of hi.
lod means that it is the gameplay version of the costume.
hi is used for the dressing room, the talking scenes, the character intro/outro at the start/end of a mission, and the shinovi tenshin. (possibly also the nippo cutscenes too)
lod version is used for everything else so gameplay in a mission AND the homeroom
anyway if I can get a fully functional model swap I'll show off a picture of that.
here I just plainly tried swapping asuka's shinovi outfit over her school outfit
EDIT 3:
I got some textures to load correctly, but not all.
and changing the texture .cat it changes both the hi and lod models. which is why the gameplay version of the school uniform in this screenshot has the green skirt from the shinovi costume:
there might be a way to change that around, I'm trying to work on it, but I cannot guarantee I can fix it without having a model export tool, so I can remap the textures.
maybe the other way around it is actually doable to swap the models, because the school uniform has less texture files than the shinovi uniform... might try that our quickly...