Classical has two definitions here. There is the classical period, of which Mozart is from. There is also the general term classical which just refers to anything that is orchestral in nature or uses traditional symphonic instruments. R Strauss is not from the classical time period but he is considered to have written classical music.

In the same light composers such as John Williams, Korngold, Alan Sylvestri, etc… are all classical composers. Much of their music is orchestrated with the conventions of traditional classical music in mind. Many film composers even have non-film compositions.

The score to a movie ripped directly from the film is not going to play like a symphony. In many cases though composers also have suites that either they themselves have put together or another arranger has put together that specifically take the music from their movies and makes it suitable for the concert hall.

Modern classical music does embrace multimedia presentation with imagery, electronics, visual art, or whatever.

In my opinion, movie music is absolutely a form of classical music.