To be clear I was not aware you were a history teacher and did not intend any of this as a personal evaluation of you. Furthermore, I wasn't saying one has to promote one's own political agenda or program students in any fashion, I'm just saying its very difficult to comment on some topics without being open to bias.
Take for instance the Abraham Lincoln election. One of the main issues was the issue of government spending on infrastructure. Do we completely avoid that topic because its controversial? Do we say it was an issue and get students to comment on this? How do we select which discussion points the students make to focus on without introducing political bias?
You'd be surprised how often people present this election as being about slavery, despite the fact that Lincoln did not support the abolitionist movement until late in the war, well after the election. But abolishing slavery is far less controversial and far less political.
Even if one decides to teach by presenting certain specific topics in a very neutral manner, and allowing student discussion to control political discourse, the very choice of topics can be politically motivated.
For example consider a 20th century American history course that chooses to divide the century into the following areas: Pre WWI, WWI, The Roaring 20's, The Great Depression, WWII, The Cold War, and the Post-Cold War Period.
Say such a course doesn't cover the war in the Philippines at all during the Pre-WWI period. There is potential bias in choosing to avoid that topic entirely. It's not the fault of the teacher who doesn't cover it. But rather the choice of curriculum. If the selection of which topics are important to teach our youth about involves portraying certain nations, individuals or political parties in favorable or unfavorable ways by selectively covering their finest moments and avoiding their worst then there is bias in the course even if it is taught neutrally.
An example of a course I took in my high school where the educator was great and but I considered the course biased was Ancient Civilizations. The furthest we got out of Europe was Egypt and Mesopotamia. We covered Ancient Mesopotamia, Egypt, Greece, Rome (Rise to Collapse). But we didn't cover other influential civilizations like the Indians, the Chinese, the Mayans, the Incans, the Aztecs, the Rus. This is despite the fact that India had the strongest Economy around for much of the time period we covered, China was the most technologically advanced for much of the time period we covered, and numerous other major factors. Instead it was make sure you know these pagan gods by both their Roman and Greek names, make sure you know these traditions many of which are nearly identical by both their Greek and Roman names, etc. Many would claim the Euro-centric choice of curriculum has bias even if the material is taught neutrally.