Desperately seeking adult role models (spoilers)
I've read people commenting here with confusion or discontent at the film's attention given to either Lisa's relationship with her mom or why the subplot with Joan and Ramon - or with her teachers and wondering why Lonergan even has two different teachers (Damon and Broderick) with semi-developed roles in the film ...
I think these observations or objections miss a key theme that I see in the film (as a unique study on the loss of innocence as transition point into adulthood).
One of our first awarenesses of Joan's role in Lisa's life comes as we hear her encourage Lisa to lie to the police about what she witnessed. Joan offers a rationale - that it would perhaps needlessly risk the bus driver's livelihood and family - and the Lisa we see at the outset buys into that stance almost on auto-pilot, while we see that she is already feeling something very powerful is awry if this is what is expected of her (to hide a truth) in the face of this trauma that has shaken her world. Her discomfort makes her start looking intensely for guidance from the adults around her, starting even with the bus driver himself at the scene of the accident as she looks rather desperately at him during the initial police interview, looking for some sign in his face as to whether she should protect him or not. She has a piece of truth about what happened that gives her a sudden unsought and unexpected power that she has no real idea how to use - either sharing or hiding that truth, she will be determining outcomes both for her own self-respect and sense of fairness and rightness in the world and also for the impact her truth will have on other's lives, initially just that of the bus driver but eventually on the lives of the best friend and cousin of the deceased, in diverse ways. As we see, when that almost-desperate quest leads her to track him down, he turns out, like most of the adults in her sphere of influence, to not be very responsible or circumspect or even honest with themselves about what motivates them. That realization decides the matter for her, as she increasingly wakens to a kind of cognitive dissonance - seeing that her hidden truth about the accident is allowing an adult to forego his own self-honesty or sense of responsibility.
Among the litany of adults she seeks out in the course of the film, one teacher (Damon) becomes the first other adult (besides her mom) with whom she shares and sounds out her dilemma. She is searching for other potential modelings of how an adult should handle such a moral-compass situation - but as she eventually 'tests' Damon, he proves himself to be as inadequate a role model in his own way as the bus driver who is blind to himself or her mother who encourages her to lie. Yes, she seduced him, but at a perhaps unconscious level, I think she was hoping and needing him to prove to be an adult with boundaries and honesty about what his role as an adult needed to be.
Broderick represents another exemplar of an adult in denial, not honest with himself even about whether he really seeks open discussion among his students when it turns out he cannot handle open debate of interpretations of literature's meanings. She seeks from afar to test whether her father holds an answer for her but he too turns out to be a bit fraudulent in his overtures of caring and prioritizing family - and she hears by phone how he seems to be in his own denial about the life and relationships he has opted for in having separated from her mother.
No one quite seems to get what she sees and struggles with as to what matters in regards her own truthtelling. So, lacking that, she ultimately trusts the gnawing instinct in her that makes her change her police report - after first giving Ruffalo as bus driver a chance to 'prove himself' as a self-aware adult but he fails the test in his own self-serving denial about what his own responsibility was. Then the best friend of the deceased, who she initially leans toward as an ally in truth-seeking, turns out as well to have her own erraticnesses that lead her to see there was only really a surface alliance albeit a crucial one that helped her search for how to put her inner moral-compass instincts into play. The deceased's cousins - as well as the lawyer who seeks to rationalize their pov - appal her with their respective myopic, hypocritical, and/or self-serving stances.
Everywhere she turns - and imho the point is made only by Lonergan having her turn to so many different adults in her life or to study them as to their role-model potential - she eventually sees that she has to figure this out ultimately on her own. And when she does, somewhat surprisingly, it is her mother who voices respect and alliance in the choice Lisa makes to tell a truth her mother had initially advised her against. It is her mother who winds up being able to engage open debate, so to speak, on a truly complex moral dilemma (whereas Broderick as her teacher couldn't countenance any challenge to his stance on even a literary interpretation). Meanwhile, the relationship her mother has developed with Ramon, a relationship we see Joan herself seeming somewhat ambivalent about, not enthralled by in a way that an adolescent in particular would expect or idealize as the way a romance would/should look, becomes yet another adult modeling of 'how to be' as she studies her mother's own 'investment' in this relationship. And until Ramon's sudden death, she mostly seems to draw from her observations of her mother vis-a-vis Ramon a further basis for feeling alienated from her mom. But then she - and Joan herself - discover how much Joan has grown to care about Ramon in the face of losing him. And, significantly, he has brought opera first into Joan's life and then into Lisa's - an art that highlights adult struggles with trauma and tragedy, singing out in grief and desperation and searching for their own way to honor their emotions (the world's great operas transmitting this emotional message in a way that hits Lisa as it hits many, without needing to actually know the language being sung).
As I see it, the film culminates in this operatic climactic moment with Lisa suddenly 'getting it' that the very quest she has been so confounded by - seeking to responsibly deal with the role she played in a tragic death, seeking in other words to assume an adult-like responsibility she's suddenly been thrust in - no longer able to be the 'innocent' who has no such responsibilities - and where her quest for role models has mostly fallen short - that very quest is at the heart of the struggle of adulthood, sung of in operas and, it turns out, something she can see as a potential bond with her mother, who suddenly emerges as the one person she can feel safe and connected with enough to unleash the grief that this rite of passage has stirred in her, quite profoundly.