My god I'm so sorry your poor DD has gone through this, the incident has been handled appallingly. Public schools have a very clear response of both teachers and upper management on how the ed dept and child protection wants them to handle it. It appears this school has attempted to sweep this under the carpet and not expected you to go into bat for your daughter. For starters they should never have been questioned together. She also deserved to have an advocate present, particularly how you specifically said you wanted to be there.
What disturbs me the most is that the school says your DD only said he slapped her and he did not admit he did anything, yet your DD is adamant she told and he admitted it. I believe your daughter. This isn't just mismanagement, it's far more serious imo, they are deliberately lying to cover it up. Omitting you from the equation shows the cover up was very premeditated. Not having you there meant there was no proof if he did confess.