So, after a little more than a week he suddenly started to wear a school uniform. I'm not completely sure why he started to wear the uniform, but it may be because he went from having grade B to grade A, since that probably counts as a promotion.
The only problem now is that he wears a different colour of the uniform every day. Sometimes he even leaves in one uniform and comes back home in another. I've tried sims.modify_career_outfit_in_cas and chose that he should have the black uniform, and a different pairs of shoes than the ones that were given to him, but the next day he wears a different colour on the uniform and not the shoes I gave him. I have never had any problems with the sims.modify_career_outfit_in_cas for sims who work, I don't know if it works differently for those in school.
I also noticed that my A-grade child/teenager Sims wear different uniforms every day and thought until now that this might be a feature, not a bug. But come to think of it, an uniform should serve as a group emblem, therefore stay always the same and not be changing randomly at will.
@Japanic2012 I created a separate thread for the uniform changing colour, so maybe you should check it out since you seem to have the same problem. Here it is: https://answers.ea.com/t5/Bug-Reports/SE-School-Uniform-Colour-Changes/m-p/10137155#M176184
