This is part of a series called Work or Break, where we'll examine a particular workplace/job search issue from 2 perspectives: an approach to work the system when you need to get or keep a job, and the approach you can take if you're in a position to help break a toxic or damaging system. Institutional change takes a lot of individual action, and not everyone is in a position to make those moves, but if you are, there's a strong moral argument for doing so.
So, you're leaving your job, and it's because there's something rotten in Denmark. Whether its a bad manager, a bad decision your leadership made, an abuser that the company won't deal with properly, or whatever else, it can be hard to figure out if you should say something.
Sure, it would be great for the company to know. If your boss sucks, HR and company leadership might genuinely not know. This especially applies if your boss doesn't suck per se but you need to have a thick skin to deal with them, or you've got to thrive in autonomy because they're really hands-off, or whatever other detail. It's good for companies to know that sort of thing, and often, the only way they could possibly know is if you tell them.
And if you're leaving because of something happening higher up the food chain, then they very probably don't know that, unless communication at your org is really functional (I'm not sure I've ever worked anywhere functional enough that those messages consistently make it from the bottom to the top, and I've mostly had good jobs at good orgs that valued hearing what staff had to say at all levels).
Today's Ask a Manager has an interesting question about exit interviews (question #2 at the link). AAM breaks down the dilemma well; if you're confident that HR will handle your feedback discreetly and there's no chance for it to impact the reference you get, then you might consider being honest. But if not, you should just say that you're going off to grad school and leave it at that.
Work the System:
You tell HR you're going to grad school, and demure on pointed questions. As Alison mentions, HR knows that turnover under this manager is high compared to the rest of the org, and you don't need to take a personal risk to help a company that you're about to leave maybe make itself a tiny bit better. And if your bad boss isn't great with critical feedback, you could torpedo your reference or destroy any future chance to come back to the company.
Break the System:
Sure, HR can tell there's something up, and investigate. But you know what investigation looks like? Asking questions, particularly in exit interviews! First off, whether you plan to work or break the system, you need to be prepared for the possibility that HR does know something is up, and might ask you specific and pointed questions to get at what you might not want to tell them. But even if they don't, you're in a relatively low risk position - after all, they're not going to fire you if they don't like what they hear - and you're adding your voice to assertively declare that something is not OK.
In a company that earnestly wants to be a good company, one complaint is probably enough to spur some developmental feedback for your boss. We usually assume that assholes know they're assholes, but that's just not the case in my experience, and if you were driving your employees away, wouldn't you want to know? Especially if it turned out that the last 3 people who "left for graduate school" really left because they hate your guts?
But even in a bad company, the metric for when that boss gets fired or forced to reform is when it becomes more painful for your company to keep them than fire them (or give them some rough feedback). Even if you're the 5th person to leave the team in a year due to random personal circumstances, the company still hasn't received any message from its staff other than "we like grad school" or whatever. There's no pain there.
If they're ignoring the problem, saying you're just going to grad school explicitly tells them "there's no problem here."
And sure, if you need to keep that reference intact because you need to get another job and eat and live and stuff, you gotta do what you gotta do. But if you're in a position to (accurately, and gently if possible) give some feedback that points to the actual things that need to change at the company, you're doing a valuable service for those who come after you.
Bad systems exist because their internal power differentials prevent the kind of collective action needed to change them.
If you declare you're going to grad school and leave, you might be making the right choice for yourself, but you're making the wrong one for everyone else. You're helping ensure that those that come after you find themselves in the exact same dilemma, with the same risks.
That boss will continue to be a bad boss until someone either fires them or compels them to change their ways. There's no magic intervention here that will make the institution a better place to spend half one's waking hours without anyone having to take a risk.
The message is much clearer when it's actually, you know, a message, instead of a lie you say while you're ducking out. Again, do what you gotta do. But if you're in a position to exert a little power here, consider taking it, for the sake of those who will come after you.