I was wondering, are they uncommon/strange? Guess it depends on the circumstances, perhaps.
They are relatively common. They aren’t too weird, but they usually come with a host of problems, in addition to the usual ones. Especially when it comes to the high school senior choosing a college.
This is one of those subjects that really has to be approached on a case by case basis when it comes to if the relationship is a good idea. I know you didn’t ask that explicitly, but I suspect it’s an underlying question.
I always picture awkward proms.
Based on my experience as a Resident Advisor when I was in college, if the college student is away at school and the HS senior is home, the relationship will end by Thanksgiving break. I saw it over and over, year after year. Going away to school causes people to change, usually faster than the person left at home. When the college student goes home, they are a different person.
If both are in the same town, it may be a different story.
What @amkngk is referring to is in fact so famous it is known as “The Turkey Drop” by some.
Well, are they weird?
Somewhat, but again, not that uncommon. As I said in the first post. They may lead to more weird moments than they are themselves weird (see prom). Why do you care so much about if other people think they are weird?
My boyfriend was a HS senior when I was a college freshman, and he was young for his grade! We got teased a lot. But he was brilliant and taking a more advanced college physics class than I was! He went off to MIT the following year. He dumped me on Christmas Eve, ha! Wow, 34 years ago today. His loss. 
They were so frowned upon at my HS that the one senior who brought a freshman to the prom was already considered something of a pariah and that action cemented his status. Not sure that’s fair, just reporting what went down.
No it’s not weird.
Risky? Yes. The only way I imagine it lasting is if abstinence is being practiced anyways, or if they’re living in the same town. Otherwise sayonara.
Not uncommon at all, especially it happens all the time because one is slightly older. Although it can be harder to keep in touch, it can work out. It shouldn’t matter what other people think anyway.