Having sex with your student even if they are 18 violates “duty of care”.

You pledge to care for them and ensure their safety at all times. Having sex with your students violates duty of care. It might not be considered a crime as he or she are 18 but it most certainly means you cannot be trusted to take care of school students. They are not minors in terms of age but they are minors in terms of being entrusted in your care. Duty of care does not include having sex with them

If 18 year old is independent adult in full capacity and you’ve met them in a grocery store then it’s all good