How can i convince my parents to buy me a new iPhone in about a week?

So my iPhone 5s broke almost over 3 months ago (it stopped charging all of a sudden). Ever since it broke, I have been asking my parents for a new one since then. However, they keep saying that I'm gonna get one soon, but it hasn't even happened. I wanna get an iPhone 8 or XR and I'm willing to put my money towards it as well; but that still doesn't convince them. You see, my old phone actually didn't have service on it because it was my moms old phone and they never put service on it for me (i got it when I was ten). So when I get a new one its probably going to have service; they think that having service on my phone will make me do bad things on it. I have never done anything bad on my old phone or on my other devices. Just because it has service doesn't mean that I will do bad things. They think that I will have too much freedom on the Internet, texting, calling, social media, etc. But that is not true. It is going to be just the same as my old phone but with service. And it is not like I'm too young for a phone; I'm 15 and in 10th grade. I'm homeschooled, so I don't have school emergencies. However, I could have emergencies that can happen when I got to sports practices, and my other activites. And I use my phone to communicate with my friends. Right now I'm currently using my moms phone to talk to my friends, but I don't get to talk to them as much because she is always using her phone for things. So can someone please give me advice or help?

Added (1). I found an iPhone XR on Best Buy and it costs $30 a month. I'm willing to put my money towards the monthly costs. I have told my parents that but they are still not convinced!

If they are homeschooling you at your age I would say that your parents are the type who would be impossible to convince. Regardless I would just take my bike to a store and buy an inexpensive tablet or prepaid phone I could afford assuming you don't already have one.

Promise them anything and do it.

Why not get an iPhone 6? That will give you everything you need for a third of the price of the XR. Save up the money by working and pay cash so that you are out from under the installment plan ripoff.

If your parents are worried about what you might do with it, let them install Teensafe or a similar app.

Who cares how much the phone costs "per month"? It costs $730 to purchase. Is that what you want to do with $730?

I think there's a lot of better ways to spend that money.

STOP asking for at least a month.

You are so focused on getting another iPhone that you have lost sight of what really matters. What is it that really matters?
From what I have seen, iPhones take amazing photos but you didn't mention anything about taking photos so there's no need for an iPhone. You mentioned communicating with your friends but you can do that with any phone, not just an iPhone 8.
You need to figure out why you "need" an iPhone 8. It is just plain greed rather than true need? Wouldn't a less expensive phone work just as well for your purposes? My teen doesn't have an expensive smart phone. It's a $150 phone on a prepaid plan. You may have a better chance convincing your parents to agree to a working smart phone that costs $80-$150 on a prepaid plan that can cost as little as $35/month TOTAL. If your parents bought an iPhone 8, they would have to sign a contract, meaning that for 2 years, they are stuck paying for the phone AND the service. That's not just $35…
You mentioned that you can afford to pay $35/month. Many carriers offer prepaid plans for $35/month (unlimited talk and text, 1-3 GB of data). You could suggest to your parents that you buy your own phone (an affordable prepaid phone) and pay the monthly fee yourself. If one month you don't have the money, no problem, the phone is just not going to work until you are able to pay.

You are at the age where begging, making promises & crying plus nagging is acceptable so the decision is yours…

Why do you have to get an iPhone 8 or iPhone Xr though?

iPhone 6s can do pretty much everything you need to do as long as you do all the basics and it only costs like less than $300 if I remember correctly. Since you said YOU would be willing to put in your OWN much into it, then WHY DON'T YOU?

The new iPhones costs like $1,000 if not more. So why on earth would you bother buying them when you know those will lose it's value, be underpowered and all that within inside of a couple years? Lol Only suckers buy them new.

Just buy a iPhone 6s as it's still a really good phone to use and probably will still be supported for a few more years.

No 15 year old needs an iPhone or anyother smart phone
Your parents should buy you a $25 flip phone and that is all you need