Teach In

Where did the term "teach in" come from? Did this come from the same sort of source that the term "sanitation engineer" come from - i.e. some person trying to disguise the true value of stuff. How much teaching actually goes on at these things?

Some faculty and students at Minnesota State had a teach in regarding the "Social Security Myth." Here is the opening paragraph from the local paper's story (subscription required):

It was admittedly one-sided. But for the people who put on Monday's Social
Security teach-in at Minnesota State University, it was seen as a chance to have
their say in a debate they think already has been a one-sided affair.

Translation: we didn't invite an opposite viewpoint because we didn't want to.