The note, published by The Times, shows Mr Belfield writing that he would "put together a list" of students with a boyfriend or girlfriend.
"These students — if in [Years 12 or 11] can expect to find new schools in September."
"School is not the place for romantic relationships — ever."
Mr Belfield later clarified his comments to say students in a relationship would not be automatically expelled, but were in danger of "academically underachieving" and would be given a chance to "review their current romantic situation".
It's not the first time the head teacher of the exclusive school that costs nearly $60,000 per year for full boarding has courted controversy. He was previously forced to apologise after saying students should not be "forced" to learn Welsh language and said the school was oversubscribed and suggested those who broke the rules could be booted out.
The comments have struck a chord with many online, with Ollie Whittingham saying: "Pupils would like to remind the head teacher at Ruthin School that it's really none of his business."
Patrick White wrote that it was a "very important lesson" for teens to learn.
"Always think how controversial emails sent to your organisation (that you know won't be popular) will look in the media when someone inevitably leaks it."
Chris Le Poidevin said: "I academically underachieved because I drowned the angst of not having a teenage relationship in alcohol. Maybe trust your students to prioritise."
Peter Newman said: I think this man rather misses the whole point of boarding school!"