The allegations included sexual encounters and "sexting" or sending suggestive text messages.
Some of the incidents are alleged to have happened in the classroom. The situation only came to light when two sets of parents complained to the school.
The teacher was stood down while an investigation was carried out but she resigned before a disciplinary process could begin.
The matter is now being investigated by the New Zealand Teachers Council.
Hamilton Police, who received a complaint on Monday, said there was no criminal offending in the case and Mr Morgan said the only difference in the situation was that it involved a teacher and student.
He believed the law was adequate.
"Because by the age of 17 most young people these days are already sexually active, it's just they are doing it with people of a similar age and fellow students, not their teachers."
In a criminal sense he said there was no difference in a role reversal of the situation, even though society might be more appalled at an older male teacher becoming involved with a younger female student.
"It's obviously a matter of grave concern for any school, or possibly the Teachers Council, but if she's 17 and she's consenting it's not an offence and that's exactly the same if it's a female teacher and a male student."
Sex crimes were those that happened with children under 12, however there was a defence for sex with a child aged between 12 and 16, Mr Morgan said.
"You have a defence if the complainant consented and you're of much the same age."
He thought police would not bother charging two underage people in a consenting relationship and if one of them was over 16 they had a defence.
"If you prove that you have taken reasonable steps to find out if they are over 16 and you believed on reasonable grounds that they were, and the young person consented."
The teacher declined to speak to the Herald today when approached at her parents' Waikato farm.
A man and woman at her boyfriend's parents' home in Hamilton also declined to comment.
Yesterday St John's College principal Shane Tong informed the parents of those students who had the most interaction with the teacher of her resignation.
He said affected students were being offered support.