The other big problem with teaching ID has had a thread here before: what would the lesson plan be? Maybe:
Tuesday. First 30 seconds of class: "Some people posit that some intelligent force did something like designing the universe or life at some time in the past. These people don't know who or when, or what this Designer did. So let's move on to the Krebs Cycle."
but how do we know there was no O2 in the air before that? I was taught that there wasn't in school, but how can we know?
We can know by looking at the minerals that are in rocks more than 2.4 billion years old. There are multiple chemical species that show that oxygen was absent in the atmosphere back then - like iron sulfides where you would have iron oxides now, uranium minerals in river-laid sands that would oxidize to different minerals if free oxygen had been around, molybdenum minerals that would never have fallen out of seawater solution if it were oxygenated like today.