I have a very tight budget, and my ex is not going to be pitching in ("I pay child support; that's enough.") I had a plan for savings for this year, but because of medical problems for me and the kids and work hours shortened that fell through.
Does anyone have any tips or advice on how to navigate this year as inexpensively as possible?
I can't give a perspective from parental experience, but I can give some thoughts as a teen in college with my senior year of high school still within sight from my rearview mirror. I would have a candid but sensitive and diplomatic conversation with her now. (I'm using female pronouns but I don't know if this is your son or daughter, so just switch it if necessary, ha). I'd feel hurt if my dad wasn't willing to contribute more for such a special time of my life, and emotionally would feel that it was a personal slight, even if logically I understood that it wasn't. It just would be painful to me. If I were in your shoes I'd vent to a friend who could be trusted to keep it all confidential to avoid implosion at the unfairness of it all, but would try to contain frustrations while in front of your kids, and especially your senior. I'm not urging you to lie or mislead her, but just to use care to explain the situation with the most necessary facts given so you can collaborate on a plan for this year, but perhaps consider not revealing that her dad isn't willing to give more. That omission wouldn't be to spare his reputation or as a favor to him, but an act of compassion for her. If she's already aware of the facts, and already knows exactly what you wrote here, then carry on.
Ask her to collaborate with you to strategize how you'll pay for expenses that are important to her this year. This is what my friend's mom did the summer before our senior year. In her case, she was fully aware that her dad was not paying a dime towards anything. He'd moved to Australia when she was 10 and had no contact with her until he called one day and told her he was remarried and she had a half-sister. We went to a very expensive private school where a lot of the girls were flippant about the costs of senior year, of both the practical expenses like for the SAT, ACT, AP exams, and college applications, and the ones that are technically optional but feel mandatory because they're not replicable, like homecoming, winter formal, spring break, prom, Grad Night, and the senior trip, but she was on financial aid. She got a job the summer before senior year and saved up for the fall. She worked at Bath & Body Works over winter break, and also got an employee discount that helped with gifts. She has one of the best work ethics of anyone I know, and is extremely responsible, so I think there were benefits to her working, as well as the satisfaction of paying / helping to pay for things herself.
The SAT, ACT, and AP exams have fee waivers or reductions, but you must apply in time for them. The Common App also has an application for a fee waiver. Questbridge helps lower income students apply to elite colleges, but if that's of interest she'll need to move fast to get everything in order because the application deadline is earlier. I'd otherwise advise against applying through the early action deadline to college, unless the admit rate for that cycle is significantly higher than for regular admission. Friends who've gotten into college through early admission have said they received less substantial financial aid packages because the colleges knew they weren't competing with other colleges, so there was less of an incentive to be generous.
If dual enrollment classes are available between her high school and local colleges, I'd definitely encourage her to consider taking them. I was able to start college with sophomore standing because of dual enrollment and AP classes. Friends who went to state universities where there's a far higher cap on the number of credits you can transfer in actually started off as juniors. Dual enrollment classes are a far safer bet than AP classes because they are not contingent upon getting a stellar score on the exam at the end of the year. Only 4.1% of students who take the AP Biology exam the year I did earned a score of a 5, but my brother's entire class got the exact same amount of college credit for the dual enrollment biology class they took.
Many communities have dress exchanges or organizations like The Princess Project where girls can get dresses and accessories for homecoming and prom. The friend I wrote about above was able to get beautiful dresses through a dress exchange an older friend is a part of at a local college. It was set up so girls could exchange dresses for sorority formals, but high school friends were able to participate. She traded in her junior year dress and mine for two dresses that were in like-new condition. I think boys just rent their tuxes, but maybe there's something for them to help out with expenses, too.
Good luck to you both!