This might seem like a silly thing to post on a forum, but frankly I'm at a loss. I've been TAing this one class for 5 semesters now, but this semester it's a new professor so things have changed, and I misunderstood one big instruction. I teach the labs and am pretty independent, so I'm not quite sure what to do. Basically, our university has a colloquium every week. The students in my class (physics I) can attend for extra points. The professor told me that the students can opt out of one lab (out of 8) and do an essay on the colloquium speaker some particular week. However, I misunderstood this to mean that instead of doing any of the labs, they can write essays over the different colloquia if they so choose. And that's what I told my students. My intention, of course, was to correct things this week (the 2nd lab), but I was appalled when only 10/25 students showed up to lab this morning because they thought they could just write essays instead! Not only that, but fewer than 30% of my students turned in their first lab report. I told the professor and apologized profusely, and he basically gave me the "It's your issue, fix it" spiel. I don't have my students' e-mail addresses, nor am I able to go in during lecture and clear things up (I have a class at that time), so I'm afraid that the ship has sailed for most of my students! What do I do? TLDR; Told my students they don't need to come to lab, but found out they do. Have no way of contacting them or apologizing or adjusting the gradebook. What do?