I'm pretty sure even just dating a college student let alone a high schooler is bad.
I’m not sure what you’re saying here—sure, teachers shouldn’t date their students, as part of the general rule that people (in real life as opposed to fiction) with workplace power shouldn’t date people they have, or may potentially have, power over.
But are you arguing that a high school teacher shouldn’t date a college student? If the issue is the age gap, what degree of age gap (discounting any official power differential) would be acceptable?
(Stipulating that I’ve known real-life age-gap couples of a number of different varieties, and that I’m generally fine with non-abusive age gaps in fiction.)