I haven't seen the movie in a few years, so forgive me for not being specific.
During the scenes where the kids are running around in the hallways, look at Anthony Michael Hall's shoes. During this sequence of scenes, he wears about 3 different pairs of shoes. Check it out.
I'm sorry, but I don't see it, all I see are the blue nike's. Are you sure you aren't thinking of Judd Nelson's character?(John Bender) who wears a different shoe when he's in the gym?
I watched the movie all the way through today, and saw that he indeed had on blue Nike's during the whole movie, although in a couple of scenes they looked older and in a couple of scenes they look brand new. This may be where there is some discrepancy.