(I'm not trying to use any of this as an argument that "female teachers having affairs with students" is okay - but the 'duty of care' concept has never been entirely clear to me...)
The official description of the duty of care for teachers in Ireland is as follows. I think having sex with the students would not exactly count as acting like a reasonably prudent parent!
The following general principles apply:
Teachers must take reasonable care to ensure that their students do not meet with foreseeable injury. They have a duty to protect the children against foreseeable risks of personal injury or harm.
The standard of care is that of a reasonably prudent parent. The degree of care depends on such factors as the age of the students.
There must be an effective system of supervision in operation in the school.
The duty of care applies while the students are on the school premises during school opening hours. It may also apply if the students are present outside of official school hours, e.g., if they arrive early or leave late and the teacher/school has agreed to the students being present
The same duty of care applies if a teacher voluntarily supervises children.
Teachers are required to follow good standards and approved practice. Risky classes and activities require a greater degree of supervision. If goggles, protective clothing, etc., are supplied for certain activities, it is the teacher's duty to ensure that they are worn
Teachers are not insurers and cannot be responsible for every accident in school hours. The courts accept that some accidents will happen no matter how well supervision is carried out and, in such cases, the teacher/school cannot be held liable.