Hi, just bumping you a bit as this board can get quiet. I'm far from an expert but I would say (as you've found), what doesn't work with teenagers is escalating consequences that they can tell you aren't really prepared or able to follow through with, then when inevitably they still act up, escalating further and further until effectively they're grounded/lost their phone/pocket money/wifi until the end of time and you're in a kind of stand-off with them seeing who'll back down first - usually it isn't the teenager in that scenario!
TBH I never really liked using grounding as a punishment, as getting them out of the house to their hobbies/see their friends regularly was essential to the sanity of the rest of us and generally they were much nicer to be around as well if they'd been out of the house rather than cooped up all day which was just horrendous, in particular making sure they got at least some physical exercise was absolutely critical so avoid punishments that prevent this. I did use taking their phones and/or turning off wifi as a punishment occasionally although again the subsequent wailing tantrums felt more like punishment for me at times!
So I would say try and pick your battles, maybe sit down with her and your other kids and set some basic 'house rules' that everyone can sign up to, don't be too ambitious, you want them to be achievable or you'll drive yourself into a tizzy constantly policing them (at one point I tried for no swearing in the house, ours were foul-mouthed as to be fair are DH and I at times 
so that turned out to be unenforceable so we settled for 'no swearing at one another or in anger'). I think it's 50% treat them like an adult so listen to what they have to say and take into account preferences, concede ground or comprise wherever possible (one of ours told us she felt so anxious if phone was taken off her she actually went into an panic-y spiral so we agreed to avoid using that as a punishment) and 50% treat them like they're toddlers (teenage brains esp when wound up are not far from functioning like a toddlers, which explains some of their behaviour!), so minimal choices only, avoid overwhelming them, strong routine, fresh air and exercise every day, simple and immediate consequences with rapid forgiveness of bad behaviour, try and make things seem like their idea not yours wherever you can, ignore tantrums, you know the drill 
.
Re school work, I wasn't averse to a bit of bribery (rewards for good exam results/reports etc) but equally 14 is quite a good age to let them experience the natural consequences of not doing their work - getting into trouble at school etc., as it's unlikely to have too much of a long-term impact (within reason of course!). By 16/18 the exam results start to actually matter so you'd get a bit more anxious if she really is falling behind but I wonder if trying to get her to take a bit more accountability for her own work rather than you nagging/arguing with her to do it might work? You could try having a grown up conversation with her about what she wants to do in the future, if there's a particular career path or college course she wants to do or even if she just thinks she wants to go to uni or do an apprenticeship or whatever, you will be able to look up the entry requirements so she knows she needs to get however many GCSEs or whatever, bit of inspiration might help her feel she's working for herself rather than for you? Pick your moment to do this of course, if she's in the middle of a screaming ab-dab about having to do her homework she won't exactly be receptive .
Finally look after yourself, parenting teenagers is tough but it does pass eventually, you might even come to miss them once they're grown up and gone much as you might willingly move to Timbucktu and never see them again right now!