to create a npc is not easy in my opinion ...
to change something on your charakter is possible, if you go through with it. The easiest change you can make is to modify the head (named Principal_FHead.png for the female/futa version and Principal_MHead.png for the male Head) .
Haircolour, hairstyle, glasses, is easy with a drawing programm, changing the shape of the face ... maybe it is easier to look through the faces that are availble for other npc, copy it, modyfy it and name it according to what it it should be (Principal_FHead.png or Principal_MHead.png).
Make a copy of the face you want to replace and save it under a other name so you can restore ist if you want to.
If you change the body you are a little bit screwed than it become a lot more of work, because you must change every clothing artikel your charakter will wear.
this is out of my memory and a quick search in my version of the game ...
the heads of the principle you can find under
\HHS+1.9.5-2050-Release\Schools\ModSchool\Images\People\SpecialCharacters\Heads
and the clothes under
\HHS+1.9.5-2050-Release\Schools\ModSchool\Images\People\SpecialCharacters\Principal_All
i think there was an article under
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how you could create your own mod. i highly advise you to do this. If you are not making a mod but are thinking of changing the original game the path will change from \ModSchool\ to \NormalSchool\ but i never tryed this.
i looked ito the files ... if you want to change the body you have appoximly 410 pictures that you must edit ...