Wife Works In A School, And Contract States That She Has To Work An 8 Hour Day, But Does Not Specify Hours

So my wife works in a school district in New Jersey. A week or so ago her principal asked her what hours she normally works, as most workers (she's not in admin, but also isn't a teacher) stay 30 minutes after the kids are dismissed.

Currently she gets in before the kids so she can prep for meetings and do other work related tasks before the students arrive so she doesn't have to scramble in order to meet her daily objectives.

What was insinuated with the discussion was that she needs to stay after school for those 30 minutes post dismissal, but her contract states that she just needs to work her required hours per week. She spoke to a union rep who told her it's an 'unwritten rule' in the contract and she should start doing it.

My wife has tenure, but she is having a hard time with this because of how it's being handled. My position is that the district can pound sand since explicit hours are not defined in the contract, just that she be in the office each day for 8 hours.

What say you?