"Despite the legal definition of an adult as someone “18 and older,” state courts have held that sexual relationships between students and teachers are against the law. In 2010, the Supreme Court of Washington ruled that a choir teacher’s sexual relationship with an 18-year-old student constituted “sexual misconduct with a minor,” even though the student had reached the state age of majority. The teacher avoided jail time but lost his teaching certification after pleading guilty to coercion...

In the absence of state and federal laws expressly prohibiting sexual relationships between teachers and students who are “consenting adults,” at least 18 years of age, some schools have instituted their own internal policies that make the practice unethical and cause for written censure or dismissal."

Student Teacher Relationship Laws: Sexual Relationships With Students Impede the Duty of Care - Lewis and Llewellyn

It depends on where you live in the US, but in several states, it IS illegal. Where it's not illegal, it may be considered unethical and cause for dismissal.

However, I noticed BigBubba lives in Europe, so perhaps the laws & ethical considerations differ.