Are education levels and crime rates inversely related, suggesting that education entails external benefits?
Yes, this is true, making education a form of a positive externality.
An externality is a cost or benefit that is experienced by someone other than the people who produce or consume a given good or service. A positive externality (external benefit, in your words) is a benefit that is enjoyed by those people. In the case of education, the producers are the schools and the consumers are the students. But we know that other people benefit. One of these benefits is that people with good educations generally have good economic opportunities and need not resort to crime.
When people are educated, they are less likely to commit crimes. This makes all of society (including people other than the consumers and producers of education) better off. Therefore, education entails external benefits and can be called a positive externality.