First, there is a difference between opened and open. Opened is the past participle of the verb. Open is an adjective, here meaning “ready for business”.
Review the difference between for and ago. For is used to talk about a duration or length of time, ago is used to talk about the start of a length of time.
The present perfect (has been opened) is not used with a specific time reference such as 25 years ago today. When we use has been open, we have a linking verb with an adjective.
Any of these is correct:
Our school was opened 25 years ago today. (If today is 31 October 2012, our school was opened on 31 October 1987.)
Our school has been open since 31 October 1987. (note: not opened)
Our school has been open for 25 years. (Note: not opened)