Getting their money's worth

My mom warned me before I ever started subbing that some principals are super picky about making sure their subs stay all the way until the end of the day, even if they are doing NOTHING. I was very careful about that at first, but after spending some time at the same schools, which were happy to allow me to leave as soon as I was finished, I relaxed and accidently got myself in trouble.

Well, in trouble might be a little strong. At a school I subbed at a few weeks ago, the bell rang, I cleaned up the room, finished up my notes, and headed to the office to check out. I thought I'd spent a fair amount of after school time to make it seems as if I was doing stuff, so I'd only be leaving a few minutes early, but because this particular teacher's schedule had two free periods at the end of the day, I judged wrong. I arrived down at the office a full 30 minutes before I was supposed to leave and was told that I was supposed to stay until 3:45. She ended up having me stand outside (in the cold and rain) until 3:45 or the rest of the students were gone, whichever came first.

This annoys me.

I get it, you're paying me for 8 hrs, you want me to work for 8 hours. But the pure stupidity of having a person sit on their ass and do nothing (or alternatively read a novel) is ridiculous. I see it more like you are paying me to do the job of subbing for this teacher, the hours aren't the deal, it's the job. Do the job, get the pay. This is especially silly because in this situation I had already asked if the library needed help, I had asked the office and they had nothing except supervising space that was already supervised. If I'm an actual teacher who has actual planning and meetings and other things to use for that time, that is logical. A teacher who leaves early every day should be making the time up some other time. But as a substitute, there is nothing I Can do to help the teacher I'm covering for. In fact, even organizing or cleaning or grading, which might initially appear to be helpful tasks, might actually create more work for the teacher. I know that I wouldn't want a sub that I didn't know to do any of that for me, I want to look at my own students work, and do my own organizing.

Yes, the school deserves a full day of my time, but if you're going to make me stay after the work for my teacher is done, they better have something for me to do.

This is my opinion, what do you guys think, am I being reasonable? Is it ridiculous or commonplace for a school to expect me to sit and do nothing in my classroom instead of go home when I'm done.

Does a sub get paid for doing the job, or working the hours?