Polly's age confuses me

I know having a set fix on time isn't really this show's main thing, but it does get me.

Betty and Cheryl seem to be in the same grade. Which puts Jason in their grade. Polly is of course older than Betty, so in a higher grade. But they never really state. When she's at the Sisters of Quiet Mercy, or the farm, or announcing plans to move to San Francisco, they never even sideways mention her struggles to finish high school while living away from her parents, with kids of her own. Logically, I feel like that means she was between high school and college that summer. But they never mention college plans either. And if she were that much older than him, that'd have been mentioned it seems, as a three-year difference at that age is much more significant and scandalous. They never mention how she has money either. Also, if she was between high school and college, she's much likelier to have been 18 at that point, meaning her parents really would've had no way of forcing her to stay with the Sisters.