So going from canon from the tv series, Dawn's birthday is in February or March.
You said yourself, they got her age wrong at the carousel - so why trust everything else to be accurate?
Alternatively Dawn's birthday is in fact September - when she was created.
Which would make her 15 by OMWF
And just turning 16 during 'Lessons.' And therefore a sophomore - which she is. If she had been 16 in the March/ Feb preceding she would be a Junior. Which she isn't.
As S1 Buffy arrives to Sunnydale a couple of months after her 16th birthday, Dawn is 'Welcome to the Hellmouth' Buffy age around 'sleeper.'
Considering that Buffy's birthday information changes twice within the 'I robot you Jane' episode - and the 'witch' episode includes a banner stating 'cheerleader try outs 1996' - despite the fact that 'canonically' she doesn't even
arrive in Sunnydale until 1997 ... pretty much any and all timelines in season 1 are suspect. The banner in conjunction with her having slain vampires for over a year would make her get called in 1995 - which we know she didn't because we saw it happen. The writers are perfectly happy to fudge around with dates, particularly from early on, if it suits them better. For example - if they absolutely mean Welcome to the Hellmouth to mean the winter solstice (though as we see they were filming in 1996 - they may have meant summer solstice, but it all just got pushed back... so we should take 'solstice' with a pinch of salt, as they just wanted something mystical sounding) then Buffy must arrive almost exactly on her birthday. (or even before - but they say she is 16, so not.) In which case if she has been slaying for 'over a year' as she says in witch ... then she has been slaying since before her 15th birthday... or it was a mid season replacement show - that didn't expect to be made in the first place, never mind picked up for a second season and they really weren't that bothered for numerical continuity as they certainly had no idea that people would still be picking it over 20 years later. They probably thought it didn't really matter - and could make off hand comments without it being scrutinised from every angle. But assuming the solstice is important - and not to be hand waved as 'solstice means something different for demons - so this wasn't January', then Buffy has been slaying 'for a year' not 'over a year' in 'witch'. If we ignore solstice - and push 'the harvest' back to March - then Buffy's statement in 'witch' could be true (just), if she was called around her 15th birthday. But they can't both be true. One of those factoids has to give.
The fact is - going on later canon, if Buffy was called as the slayer in Jan 1996 (so the very beginning of the second semester of her freshman year), and began to cut class, start fights, lose all her friends, burn down a gym - go to a mental institution and get expelled ... she did not have time to be the Prom princess, the fiesta queen and have the yearbook (which wouldn't come out until the summer) be 'the story of me.' Plus - she was already a cheerleader before she was a slayer. She would have had to make the cheerleading squad practically the first week of her first year at Hemrey... we don't have school cheerleading squads in England - so I don't know how likely that is ... but sophomores other than Buffy are trying out in 'witch' - which suggests that only exceptional freshman would make the cut, if older girls are still waiting. And whilst Buffy is a great slayer - she isn't on a cheerleading level with Amber (who turned down the Laker girls and
still is having to audition for the sunnydale squad).
If Buffy has been a slayer for over a year, then why did Angel wait until Sunnydale to help her? He was specifically called to help her out ... she could easily have been dead within that year when he was hanging around SunnyD. Kendra only lasted a year. His entire redemption mission would be a non-starter. But if he expected her to turn up in Sunnydale within a few months of getting called - that makes much more sense.
If Buffy had been a slayer for a year prior to her arriving in Sunnydale, then her relationship with Merrick would have been pretty well established. It would take until 'surprise' for her to have been Giles' slayer as long ... her relationship with Merrick would have been as established as her relationship with Giles during 'surprise' - and yet she never mentions him, his death seems to have not have affected her. Faith is massively cut up about the loss of her original watcher - but Buffy? Nope.
The fact is, plenty of the canon of later seasons contradicts season 1 - or makes things said in season 1 seem unlikely. Considering the later seasons are part of an established show, that is building a richer world episode by episode, whereas season 1 is ... season 1 - the later stuff is what we should accept as canon (as seen with the many ages - and sires - of Spike). Considering season 1 has date related bloopers visible on screen, anything they say there about time should be taken with a pinch of salt, as the writers have not settled on their canon at that point (and don't expect to have to - mid season replacement, shoe string budget.)
But what Buffy did or did not get up to in her mid teen years has got nothing to do with the fact that Dawn is still well within her rights to be pissed off at the way she is being treated. Dawn isn't Buffy. People go through different things and mature at different rates. One person is not unentitled to be upset about a trauma that happens to them just because someone else has it worse. That's just a race to the bottom of victim olympics. Everyone is entitled to their feelings. Dawn has had it tougher than average and is acting out because of that. That's completely normal.
Faith has had it tougher than average - and she kills people for fun. Fans still like Faith. Fans still defend Faith - and some even attack Buffy for not being kinder and more understanding, or giving her a room in her house. Getting a bit screamy because your emotions are too much to handle is hardly comparable in the villainy stakes.
