Just give them all zeros on the "pop quiz" they missed by not showing up.

That sucks because now they ruined it for everyone else too

I read this while taking a break from getting ready for a three hour lecture I have to give tomorrow. Pro tip; 45-50 minuet before a break... And engage the students so you're talking with them, not at them

Test them on the part that they all missed.

fail them. seriously fail them.

This is why we can't have nice things

YDI. Speaking as a teacher, if one or two students leave, that's just the luck of the draw. If *every* student leaves, then the only thing they have in common is you. Turn your pointing finger the other way and ask what it was about your lecture that made them not want to come back. I assume you're using "lecture" as a synonym for "class" since you're from the commonwealth, but if you meant "lecture" as in "stand up there and yap for three hours" then jesus christ, of course they left, no one can listen to someone talk for that long, even if it is on a subject they enjoy: break your time up with some discussion or activities or call and response or something. Even if you're teaching a 4 or 5 thousand level class, lecture has been proven many times over to be the least effective teaching tool even for highly motivated students. It's just too easy to zone out and not pay attention when there's no visible means of assessing whether or not you're on task. That's assuming of course that you give a shit. You're a college professor and not a high school teacher, so you have no supervision. It's not like your school's funding's tied to attendance, and you get paid whether or not anyone shows up to your class, so knock off early and go do something you like.

Is this the first time you've done this? In most classes you don't get breaks so if you used ambiguous language (like "let's pause there") the students may have thought you were ending class early. The one teacher I have that does this usually leaves us with a question to think about on the break and discuss after. If that isn't the case and they intentionally left, don't bleed your heart out because you can't make them care.

Maybe incorporate two mini breaks to keep them engaged.

Most people can't truly concentrate after 40 minutes so any lectures considerably longer than that are a complete waste of time. Labs/practicals are different cause you don't actually have to focus the whole time. A lot of people seem to be suggesting you should take attendance and penalise people but I hate that policy (luckily for me it doesn't really happen in the UK). For one thing it unfairly affects people who are ill or having a tough time and it's not like we're guaranteed to fail (I've been on the 2:1, 1st border my entire time at uni and I've probably only managed to attend 50% of my lectures through ill health). Anyway next time make it clear that it's JUST a break and maybe try being more engaging (such as asking the class questions or their opinions).