What he did was just this side of sexual harassment - which is why his school treated in that way. I don't know why this is so difficult to understand, but you don't start "flirting" with someone that you've made no other contact with. You don't start talking about a woman's body/appearance and how it pleases you as an opening gambit.
And it's hard to know for a woman if a woman is interested in that way because overwhelmingly people are heterosexual. That's not a good example. Yes, it can take some time, and baby steps, to ascertain whether someone is just being friendly or more, but that's why you spend time talking to them, getting to know them, observing their cues and body language, and then and only then float something that indicates interest and see how it lands.
Men have been messaged that women exist for their pleasure and enjoyment and that they don't have to pay attention to their cues but then they end up getting into trouble for it because that's not the way the world works anymore. Women are largely sick of that shit and aren't going to stand for it anymore, and that's a good thing.
Studies have shown that men tend to overestimate a woman’s interest in them, based on what she is wearing and how interested he is in her. However, when researchers prompted study participants to actually pay attention to emotional cues such as facial expression and body language, they did a much better job of accurately gauging interest.
When men look past their cultural programming that attractive women are there for their pleasure, and actually pay attention to what the woman wants, they are quite capable of grasping if their interest is reciprocated or not, but they don’t always make that effort — in part because the culture doesn’t require them to - although a lot of women are pretty fed up with that and not just accepting it as inevitable the way that they used to. They want to be able to go out in public places without running the risk of being treated like a cake in front of hungry children. We'll never have actual equality until that happens.
Asking someone out isn't a crime, but walking up to someone cold and saying "You're pretty, I want access to that" right out of the box is pretty close to being one.
Watch this clip of a woman walking around New York, just minding her own business for some much needed perspective.
https://www.youtube.com/results?search_query=woman+walking+in+new+york+getting+catcalled
