My youngest is almost through the teen years. Yes, these have been expensive years, but we'd planned for them and were pleased with the balance we achieved between needs and wants:
Senior pictures: I know of one girl whose mother paid something like 1K for her pictures. ONE. Assuming you're talking about public school, you'll have an option to have "just one yearbook shot" made -- it may not be highly publicized. However, I love the pictures I have of my girls. Of all the things I wouldn't skip, senior pictures are at the top of the list. A couple pictures won't break the bank, and no one ever says, "I shouldn't have wasted money on pictures." or "I just hate looking through photographs and remembering the past."
Class ring: Our school uses Jostens. Note that deep in the ring booklet among all the choices-that-all-look-the-same is a BASIC RING that offers the choice of silver or gold ... your school colored stone ... and your date /school mascot on the side. I'm pretty sure it was $99. You can get something similar from Walmart or JC Penny's, but I don't know about the quality. I loved my class ring, and my kids love their class rings -- and, yes, they still wear them in college. In contrast, no one here seems to care about college rings. I probably know half a dozen men who wear them -- and no women.
Yearbooks: Again, this is something I'd want my kids to have. It's a memory of the friends with whom they grew up. Usually they're cheapest if you order in August.
Homecoming, Semi-formal, and Prom: I gave my girls one budget for small dances and a larger budget for prom. No problem sticking to it.
Cap and gown, announcements, etc.: Again, I gave my girls a budget and told them they HAD TO buy the $9.95 cap and gown, and they could buy whatever other stuff -- scrapbooks, extra tassel, senior tee-shirt -- they wanted with the rest. Both bought JUST the cap and gown and kept the rest of the money.
SAT fees and college application fees: These are tough to get around. I did skip some college application fees, but it isn't something everyone can do -- it kind of has to do with who I am. Our state has a free application week in November, but it's mostly the small, private schools with lackluster reputations. One family I know (who has twins) told their kids they'd ONLY pay for two college applications each. Their reasoning: The kids need to be selective up front and make decisions ... instead of what most really smart kids do, which is apply everywhere, and then decide whether they like the school. Kids have this idea that applying to lots of schools increases their chances of scholarships; overall, I haven't found this to be a worthwhile effort -- the vast majority of scholarships are not school-based (of course, as I say that, my youngest IS on a scholarship from her school!).
Another expense that senior parents (well, this is actually more a junior parent thing) don't see coming: Visiting colleges. It's a big decision, and you really should visit the school. If you can't afford to visit, it's a hint that getting there/back will be a hassle for the next four years. Anyway, visiting isn't free: gas, meals, perhaps a hotel room.
No one's mentioned the biggest single teen expense: Car insurance. We chose to pay. We wanted our kids to gain experience driving (and you don't become GOOD at driving 'til you spend some time behind the wheel on your own) while they were still living in our house.
Our kids didn't work during the school year. We feel that the teen years are their one chance to be fairly sure they can make the sports team, their chance to take part in social and service clubs, and just to be kids. We told them that their JOB is to get a good education and earn scholarships -- they're BOTH in college this year, and after scholarships my total educational outlay for fall semester was $330. Yes, that's for BOTH girls. They learned the value of a dollar because we gave them small allowances, and they had to learn to scrimp. They do more grocery shopping than I do now; I can give them my credit card and a list, and food appears in my house. It's wonderful! They also learned responsibility, team work, and all those other things that kids get from jobs. They did "their job" well in high school, and I'm not at all unhappy that they didn't work at paid jobs.