I’ve had a stable position for a few years now, but before that, I spent a lot of time subbing. How good or bad a day I had mostly depended on how good the plans were that the regular teacher left for me. With good plans, I’d teach the lesson, and the students would work, and everything was fine. With bad or nonexistent plans, though, all I was was a babysitter, and they didn’t pay me nearly well enough for babysitting. It got better with time, both as I learned which teachers left good or bad plans, and as the good teachers learned that I could be counted on to follow the plans (lots of subs, even with good plans, will still just sit and read the newspaper all day, or the like).
One advantage to the job (at least, if it works like it did for me) is that you can pick and choose your jobs, and you can very easily take a day off if you want. The way it worked for me was that most of the school districts in the area used the same system for assigning subs. I only signed up with four or five of them, for reasons of travel distance (the fifth one was close enough, but I eventually realized that every day I had there was a bad day). Every morning, I’d get up at 6:00, and log onto the site showing the listings (occasionally, I’d get an automated phone call between 5:00 and 6:00, in which case I hung up and logged in then, because the phone interface was hard to use). If there were listings, I’d pick one I liked, and start getting ready for my day (or sometimes, I wouldn’t like any of them, and just ignore them, but you don’t want to do that too often, or the system deprioritizes you). Or, if there were no listings, I’d just go back to bed and sleep in. If I knew in advance that I wanted the day off, for any reason, I could just schedule that in in advance, and then it wouldn’t deprioritize me for not taking a job then.
If the regular teachers like the job you do, you can get listed as a preferred sub. In that case, the system would show you (and any other preferred subs) the job 10% earlier than it would for everyone else. So if the opening came with an hour’s notice, it’d list it for the preferred subs, and then for everyone else 6 minutes later; if the opening came with ten days’ notice, it wouldn’t show it for everyone else until the preferred subs saw it for a day.
I had a bit of an advantage, in that I preferred the math and science positions, while most subs preferred not to take those. So even if someone else was quicker than I to wake up and log in, they usually left those for me.
At the end of the day, I’d write a little note to the teacher, saying how the day went: If we got through the whole lesson, and if not, how far we got, and any good questions from the students, and any discipline problems, and so on. I don’t think that’s strictly required (some subs just write “Great day, kids were good” as their entire note), but it’s good form.