To add on to
>>2, I don't necessarily think your age is the main issue here (some people do have a solid stance on their future at that age, and if you do, more power to yah! I certainly didn't).
See, I more have concern with the amount of time you've been together. 5 months is nothing, really. There's so much you should learn about a person and your romantic compatibility with them that just isn't likely within 5 months. Especially while also in high school. It sounds incredibly romantic to be proposed to at prom and would certainly be a cute story to tell, but is it the best idea? Not really. Not now.
Like... have you guys discussed where you'd want to live as married adults? Have you even lived together to see if your lifestyles mesh well? Have you discussed finances and what to do in times of hardship? What about the possibility of children? Pets? Religious beliefs and political opinions? What you envision your future together like?
These discussions are incredibly important to have. And if you have had them and you've already figured this all out, then go for it. Who cares about getting anybody's blessings at that point.
I've been in several relationships, almost all lasting at least 7 months. And in fact, in one of them, we were together for nearly 3 years and had discussed marriage and it seemed like a sure thing for us. But things changed. And that's okay. You live and you learn.
You know yourself and your relationship far better than I do, so I can't tell you what is best for you. You have to decide that for yourself. Give yourself time to think and discuss things thoroughly with your boyfriend. Good luck and I wish you two the best!