General synonyms:
* Secret
* Confidential
* Restricted
* Private
* Top secret
* Hidden
* Covert
* Hush-hush
* Closeted
Specific synonyms:
* For information: Secret, confidential, privileged, restricted, classified, embargoed
* For documents: Confidential, restricted, sealed, internal, private, secret
* For military: Secret, top secret, classified, restricted, sensitive
* For advertisements: Categorized, sorted, organized, grouped, arranged
More informal synonyms:
* Under wraps
* Behind closed doors
* Not for public consumption
* Top secret
The best synonym for "classified" will depend on the specific situation and the tone you want to convey.