First, I think the shuttle is a real plane, but not a spacecraft, and it was used as a zero-G simulator for filming "space" free-floating for both "shuttle missions" and for the "ISS." Whether or not it ever went into orbit, it was a real craft that took off and landed, since those were witnessed by many people.

It's possible that the Challenger disaster wasn't planned, but was an actual accident-- however, since the crew wasn't actually on the shuttle when it took off, they had to fake their deaths to cover up that lie. Whether or not it went to space, the shuttle really did take off powered by giant rocket boosters, and it's possible that something went wrong with that flight. Since they couldn't publicly admit that the crew wasn't really on the shuttle, they had to then fake the deaths of the crew so that no one would catch on.

It's also possible that the whole "disaster" was planned and it was all a giant TV drama like Apollo 13. It seems more believable when things occasionally go wrong, and having disasters results in all kinds of media attention on NASA if it's fading from the public view. They could use the disaster to push for more funding, saying lack of adequate funds made it happen, or just use it for more attention on future missions.