My dd is year 13, and has been very stressed recently, but I wouldn't say that she is unhappy - quite the contrary, actually.
She is applying for medicine so the whole process has been pretty full on - open days, admissions tests, A-level mocks, EPQ deadlines, work experience, voluntary work, getting her personal statement written etc. It's a heck of a lot to think about and it all feels quite high stakes for them.
However, she also has a very active social life, plenty of time with her friends and boyfriend, two part time jobs that she really enjoys and a hobby that she loves, so overall, I think life is pretty good in spite of the stress?
Most of her friends have been quite stressed recently too - especially the ones doing early UCAS applications for Oxbridge/medicine etc. I think it's pretty normal at this stage, really, but they are all still coping.
I guess the question that you have to consider is whether the level of stress has got to a point where it's affecting their ability to enjoy normal teenage stuff. If it is, then maybe they need some sort of intervention to help them manage it better. If not, then let them crack on and just accept that a degree of stress is probably inevitable for this age group.