Z scrunched her nose at Greta's threat to involve her Head of House. She wasn't breaking enough rules to warrant Saporta (not that he'd do too terribly much to her if Greta did bing her in), also, she was pretty sure Greta was just talking. There's no way she'd actually get Z in trouble here.
"Well, that's an interesting story," Z said in response to Greta's question. "I did get a lovely new Cleansweep for suffering though two years of American magical education. But there was a freak accident and it broke and it absolutely did not involve me accidentally crashing into my aunt's garage in a game of aerial copycat with one of my cousins and having to jump off my broom and onto the trampoline to break my fall."
Z was very carefully not meeting Greta's eyes while she told her absolutely hypothetical story. She knew how Greta felt about Quidditch and proper broom care.
"For a variety of reasons my parents have elected not to replace my broom. There might have been something to the effect of 'no new brooms until we can trusted with one.' Not that I'm not trustworthy," Z was quick to assure. "You know I take excellent care of my things. My parents are just being strange, I expect it's something to do with living in the United States."
Z considered the ultimatum her parents had laid our after she broke the last Cleansweep. They weren't going to get her a new broom until they were sure she could handle it responsibly. Z wasn't pleased about this, because life was much more convenient (and fun) when she had a broom. When she was younger, refusing to get her something just because she wanted it (or broke it or lost it and wanted it replaced) was a fairly effective method in reining Z in and teaching her to appreciate what she had (except when it backfired and she decided she simply had to have something else). As an adult, it was more obnoxious and less effective. Z had just about saved up for a new broom. However, she might be able to get her parents to relent and either get a second broom-now that was an idea-or use her savings for something else.
"My parents are overly cautious and frankly, a bit ridiculous after the United States, there were so many muggles everywhere and it made keeping magic secret more of a challenge than a should be. Really, the Ministry and US Department of Magic ought to loosen up the rules. Anyways. They want assurance that I can be a responsible broom owner."
Z paused and then just decided to go for it. Either Greta would agree or she wouldn't and Z's charm probably wasn't going to work as well with this particular Hogwarts staff member.
"It possible that my parents might even like a note from someone at Hogwarts saying I won't get into freak broom accidents. Or take a broom apart to figure out what charms are necessary and to see if I can make it go faster. Or that I won't get into another broom joust with my brother and be so dedicated to winning that I plow both of us into the ground and then break my broom."