1. A school's uniform apparel supersede an individual's deals. Meaning a student athlete cannot wear a competitor's branded apparel (shoes, hats or jersey's) during game settings, practices, or team events.
2. A player may wear their individual sponsor's gear outside of games, practices and team events and promote them on social media.
3. A footwear exception does exist but has to be negotiated with the school and the competing brands where a player can wear their sponsored footwear as long as it is not branded and doesn't display the competitor's logo. This is entirely up to the school, and the two companies involved and usually requires a lump sum payment from one brand to the other. (Cooper Flagg was a New Balance endorsed player at a Nike school and it was negotiated between the three parties for instance).
4. Apparel deals are exclusive. Wearing and promoting a direct competing brand is a breach of contract.
Can Stokes still go to Kansas, absolutely. He probably will. But Kansas, Nike and Adidas will have to all come to an agreement of some kind for him to be able to and NOT violate his Nike deal. Summation, Kansas and/or Adidas can pay Nike to allow Stokes to wear Adidas gear. Kansas and/or Adidas can buy out Stoke's Nike contract completely, Nike can pay Adidas or vice versa, for Stokes to wear unbranded logo-less shoes while he plays, or Stokes can work out a buyout with Nike and end his Nike contract. This isn't the first time a competing branded player went to a competitor school (if he goes there). There is legal precedent for it. Stokes can 100 percent go to Kansas, but suiting up and playing for them while still being Nike sponsored does have hurdles, but nothing money can't fix. Unless Nike refuses to negotiate at all and want to retain Stokes, then things can get hairy.