It’s been a long, long, long time since I’ve watched Ballad, so I’m a little fuzzy on some of the particulars, but yeah, Will’s speech to Rachel awkwardly shoves him into the role of Knowledgable Authority Figure—I guess in order to re-establish some ostensible boundaries through normative teacher-student, adult-teen, male-female hierarchies. Which, obviously, Will is terrible at doing.
I think one of the things that always interested me about Ballad was the extent to which the text undermines the message—sure, you’ve got Will telling Rachel that she can’t have a crush on him, but then you’ve also got Will singing “temptation, frustration/so bad it makes him cry” in a song that’s supposedly about Rachel being too young to date. So, for me, Will’s “don’t have a crush on me, and by the way you’re amazing” speech is subverted by how conflicted he still seems about Rachel’s crush. That conflict ultimately undermines the boundaries he’s feebly attempting to set and throws his legitimacy into question. He knows what he needs to do, and he does it, but he’s clearly ambivalent about it.That tension doesn’t negate the other (sexist, etc.) problems with his speech, but it does highlight Will’s fragility and fallibility as a failed authority figure.
A better show would’ve been interested in exploring why Rachel and Suzy were so attracted to Will, and Will’s ambivalence towards Rachel, which imo stems from his over-identification and emotional investment. It’s easier, though, for Glee to attempt to frame those relationships through more reductive and familiar teacher-student tropes, which, as you’ve observed, tend to victim-blame the teenage girl(s) involved.
And that, in a nutshell, is why Glee is simultaneously infuriating and fascinating: lazy storytelling that almost always takes the easy way out, but, despite itself, retains palimpsests of complex and compelling character relationships.