Thank you everyone!
I work at a state school (not in retail, thank goodness) and love the aspect of my job that entails direct service to the students. (In fact, I'm likely to continue doing that PT for a while after FI because it's my calling.) I know I can be a good manager, even getting work out of people that everyone else had given up on (but that takes a lot of work). However, since I'm more of an introvert, managing is tiring at the best of times, much less with an "employee who you can't discipline and who doesn't have to follow any rules" (to quote JLee).
Ricky, the job is supposed to reduce some tasks by 25% so that 25% supervision can be added. However, the reality is that I'll be working more than 40 hours/week (something I could handle for three years, but - like andystkilda - not more).
neil and mxt0133, great questions about workload. The way it's set up now is that my co-worker and I have separate areas of focus so it's very clear who is delivering and who isn't. Therefore:
1) If I do not take the raise, I will not have to take on the co-worker's unfinished work.
2) If I do take the raise, I'll be responsible for all of the deliverables. And, even though I won't be dinged for it by my boss, I'll still have to interact with the fact that all that could/should be done is not happening. And that could, as Reepekg wisely pointed out, bring me down.
Cwadda, I've been invited to apply for other jobs at the school so that's definitely an option.
Vikb, I think the situation is either going to be #1 or #3 - I'll know after the meeting next Friday.
MDM, the divergence will be that the work will still not get done, but this time it'll be me who has to nag this staff person about it (rather than the supervisor who currently does it, albeit to little/no avail) and find ways to get it delivered (either by being a patient and compassionate hardass (which is how I got results from previous underperforming staff) or doing it myself.
I wish I could be paid more for "just" taking on a few more tasks, but not supervision of this person (basically the side hustle approach suggested by thegloblinchief, except the "side" hustle would be at my current job). That way I'd make it to FI faster, but minus the supervision headache. However, the only way to get paid more is to add supervision (or budget management or IT) to one's job tasks.
Thanks again everyone. This has been an extremely helpful conversation.