In one episode it was established that Bea and Clara were on the girl's basketball team together in high school. Bea's knowledge of basketball was seemingly non-existant when she first meets Opie in the first episode of TAGS.

Nobody's perfect, including Mayberry's production staff.

Actually, the reason is more interesting. Frances Bavier was an alien from another galaxy. When she came to this planet she inadvertently caused a time warp that caused the later episodes of TAGS to be filmed before she and Clara were in high school. I think she was only 134 of our years old at the time. On her world that age is considered to be childhood.

Anyway, they filmed about half the later episodes before the time warp smoothed out and then when they filmed the first episode Miss Bavier was her true age of 1450 of our years and by then had forgotten that she had learned to play basketball when she wasn't in high school.

I hope this helps you.