Barclay said she thought Belfield's rule was an extreme response and questioned how effective it would be.
"From what I understand of young people, these kinds of rules aren't going to stop them having relationships," she told Larry Williams of Newstalk ZB.
Adoloscence was a time of experimenting with relationships and understanding what it meant to connect with other people.
Banning relationships would cause resentment and make students be secret about their romantic affairs.
An alternative could be a no PDA rule, or a ban on "public displays of affection", such as hand-holding or kissing, she said.
Helping adolescents navigate relationships and teaching them healthy boundaries was important she said, and banning them could stop that happening.
"It's the happiest time [being in an relationship]. There is excitement and it does becomes a focus."
Having a relationship could distract students and affect learning, but that couldn't be stopped, she said.
"These kinds of rules won't change what they do."
The biggest risk of threatening youths with expulsion was that they would take more risks and be less safe in their behaviours, she said.
The rule would result in more negative effects than good, she said.
Belfield had previously spoken of expelling students because the prestigious school is over subscribed and last year banned students from going to the park or restaurants, ordering takeaways, smoking and drinking – regardless of their age.