Is it Appropriate to Hold a Class Lecture at an Ideological "Teach-In"?

No, it's not. Is there a plausible argument on the other side? This issue arises from an article about the feud between Joel Beinin and David Horowitz:

[Horowitz] is incensed that in 2003 Beinin held a class lecture at a "teach-in" against the Iraq war in Stanford's quad. The lecture that day on the Gulf War happened to be relevant, Beinin said, but was "definitely an act of solidarity with the teach-in." "This is as close to the line of putting politics in the classroom that I've ever done," he said. "I don't hide my opinion."

But I don't think the issue here is whether, or to what extent, a professor should hide or express his opinion in class. The question is whether it's appropriate to require students to attend an ideological event to see the professor's own scheduled class lecture. Put another way, Beinin may not have put politics in the classroom, but he put the classroom in politics.