According to the Copyright laws they probably don't legally have a hold on it anymore.
I found this on this page: http://www.copyright.gov/circs/circ1.html#hlc
www.copyright.gov wrote:Works Originally Created and Published or Registered before January 1, 1978
Under the law in effect before 1978, copyright was secured either on the date a work was published with a copyright notice or on the date of registration if the work was registered in unpublished form. In either case, the copyright endured for a first term of 28 years from the date it was secured. During the last (28th) year of the first term, the copyright was eligible for renewal. The Copyright Act of 1976 extended the renewal term from 28 to 47 years for copyrights that were subsisting on January 1, 1978, or for pre-1978 copyrights restored under the Uruguay Round Agreements Act (URAA), making these works eligible for a total term of protection of 75 years. Public Law 105-298, enacted on October 27, 1998, further extended the renewal term of copyrights still subsisting on that date by an additional 20 years, providing for a renewal term of 67 years and a total term of protection of 95 years.
Public Law 102-307, enacted on June 26, 1992, amended the 1976 Copyright Act to provide for automatic renewal of the term of copyrights secured between January 1, 1964, and December 31, 1977. Although the renewal term is automatically provided, the Copyright Office does not issue a renewal certificate for these works unless a renewal application and fee are received and registered in the Copyright Office.
Public Law 102-307 makes renewal registration optional. Thus, filing for renewal registration is no longer required in order to extend the original 28-year copyright term to the full 95 years. However, some benefits accrue from making a renewal registration during the 28th year of the original term.
For more detailed information on renewal of copyright and the copyright term, request Circular 15, "Renewal of Copyright"; Circular 15a, "Duration of Copyright"; and Circular 15t, "Extension of Copyright Terms."
If I'm reading this correctly that means they would have had to file and extension twice at least.
On top of that part of copyright law states that you fight it as soon as you're aware of it. I'm sorry, but you're going to have a hard time explaining how you didn't notice for over 20 years.
The last and final note is that I'm pretty sure Paul Allen will fight this all the way. There's just no way that Texas school is going to be able to win a legal battle against him.
But it's fun reading a little bit of wisdom in that thread and watching everyone else jump all over them.