Because teachers are in a position of authority over their students. The law doesnt want a teacher having a sexual relationship, or any romantic relationship at all, with a student. College professors even get fired for it.

Why are teacher relationships banned?

A sexual relationship with a student could prevent a teacher from carrying out their legal duties to protect students from sexual abuse. When it comes to children under age 18, the law and court opinion are unambiguous: minors cannot give consent to sexual conduct, and any such relationship is expressly forbidden.