Both are grammatically correct but carry slightly different meanings. "Has been open" means it was opened XX years ago and is still open. "Has been opened" means that the action of "being opened" is completed. It is not unlike a passive form of "I/He/She opened it" in meaning.

So, for your situation "has been open" is the correct form to use.

Another way of presenting the good explanations below of the context of the statements is as follows.

'How long has the school been open ?' - 'This school has been open since 1990.'

It was opened in 1990 and has remained open since then.

'Does the school open every year for this event ?' - 'This school has been opened [for this event] [every year] since 1990.'

Each year since 1990 the school has opened [for this purpose] and then closed ; it is a recurrent event.

Thank you for your comments. My son's English teacher marked him down because he wrote "has been open". I felt it was correct and wanted clarification.

Thanks again Daniel

Both “has been open” and “has been opened” are present perfect. “Open” and “opened” in this construct are both adjectives, not verbs. They are both the opposite of “closed”. The present perfect is indicated by “has been”.

The difference between “open” and “opened” is in the meaning. “Open” describes a state. “Opened” describes an action.

Examples:

“The jar has been opened.” (action)

“The jar is open.” (state)

“The school was opened in 1990.” (action)

“The school is open.” (state)

“The school has been open since 1990.” (state)

It’s worth noting that “has been opened” doesn’t necessarily mean that the thing in question is in the state of being open right now. When you say “The jar has been opened”, it could very well be that someone opened the jar, ate from it, and closed it. This is why it has to be “The school has been open since 1990”. The sentence is about the state of the school.

I researched and I believe that "open" here is a subject complement, and not a participle, which would be opened.

I was confused because I know the construction of "has been opened" is how the present perfect should work, but it is not what I would say. I would use open, which I found to be labeled differently. (subject complement.) I'm not an English major...perhaps someone more scholarly can tell you why this is so.

During quarantine the schools have not been open.

In your example, the school has been open since 1990.

simple past would be

The school opened in 1990.

There are cases to use opened, but I do not believe that is what you need.