It's established that Jason and Polly were classmates. However, Polly is Betty's older sister and Jason is supposed to be the twin of Cheryl who is in the same grade as Betty.
When Betty receives the call from the unknown number (The Black Hood) her iPhone rings audibly. When it shows a closeup of her iPhone the mute switch is down, meaning the phone is on silent.
Throughout the series, it's clear that the actors/actresses portraying teenagers are actually in their 20s+. This is usually done to get around child labor laws, that won't allow teenagers to work full time on a set due to interfering with their real-life schooling. Plus casting adults adds to the benefit of better acting from the actors/actresses due to having more experience plus the producers are able to sexualize the characters without consequences. Lastly, it gives the general viewing audience who are teenagers (or young people) someone they can look up to.
Polly's pregnancy belly changes through out seasons 1 and 2.