Ah, from one teacher to another, let me just say I completely understand.
I am an elementary school teacher, just started my first full time position. Last year I taught part time in my own class, and subbed, and my first year I subbed, and also subbed long term for a teacher.
So, I know how you feel. I could tell you stories. Maybe I will. I just need to gloss over some details.
First of all, yes, even the little ones will be wilder for a sub than for their regular teacher. There are lots of reasons for that, and NONE of them are personal... unless you start yelling, and then it can be personal. It doesn't start out that way though. One of the biggest reasons they get that way is the change in routine. Routine is everything, and a new face, even attempting to follow things through the way their regular teacher would do it is bound to cause disruption. Kids are like any other person. We like to know what's going to happen. It makes us feel secure. Something different makes us feel uncertain, and in kids, it means all bets are off. If you don't (and, to a certain extent, you CAN'T) follow the exact routines, then they aren't going to either. This isn't personal... and it's certainly not your fault, and not acceptable either. It just... is. Another reason for it is they are testing you, to see what you will do. Will you yell, will you cave, will you enforce the rules... what DON'T you know... that's a big one there. They love to find out the holes in your knowledge of the school and exploit them. Yes, even the little ones, and the big ones have hormones and attitudes pushing them even further.
Subbing is a tough, tough job... and every teacher who's done it, knows it. In my experience, admin and staff are very supportive of subs... well, I can think of one exception, but that's one day out of, oh, more than a hundred.
So... you have my utter sympathy. I'm kinda having a rough time of it myself, so I do apologise if I come off unsympathetic. I'm really not.
I can think of no fewer than 5 separate occasions when I was driven to tears because of the behaviour of my students.
I have been insulted, cursed at, threatened, and (today!) punched in the stomach (and this was at my regular school. Little fella with autism. Not his fault, and I'm fine) I had a brawl break out in one classroom amongst 6 and 7 year olds... and they meant business too, I can tell you. I saw a boy race across a room and punch a girl in the face. I had someone steal a lunch bag, thinking it was a purse, with plans to divy up the money afterschool. That was quite a day. I've had children throw temper tantrums and have to get the other students out of the room while they destroyed the classroom. I've had absolute chaos reign. (Again... today!)
Keep in mind... I teach elementary. In my area, that's grades primary to 6. So... 12 year olds.
What a sub goes through is not much different than what a new, regular teacher goes through at a rougher school, so take heart in that... it's not you. The problem for subs is that the trial period never ends. If you sub in the same school a lot, it gets easier, to an extent, because they get to know you.
And here's some advice for you, though totally unasked for. Feel free to ignore me, but it might help if you end up subbing again. I don't know your plans in that regard.
1) Don't yell. Ever. There's several reasons. It raises your blood pressure, as you said... and makes you miserable and angry and frustrated. It sucks. It ALSO makes the kids resent you, and hostile. It does not help control a situation. It is a way of an adult dominating a child... and that is not a good way to manage a class. If you yell, they shut down internally, and you don't have a hope of reaching them.
Instead: Speak quietly. Use non verbal signals to get their attention. A hand in the air, a clapping pattern, lights off (though, sometimes that really does NOT help). Any other strategy you can think of. If they're being disrespectful as a large group, point out and reward the ones who ARE listening. Whatever the consequences you've decided to impose on a misbehaving student, don't take them back if they back off. Give them a set number of warnings, then follow through. If you can hold them in for recess or lunch, or after school, tell them that you will, and do it. If they see that you mean what you say, it will be MUCH easier to manage them. Two days ago, I had to enforce a consequence for behaviour. They lost a significant chunk of time at lunch. Yesterday, they were angels, because they knew I meant it when I warned them. As a sub, it's tougher, because you don't HAVE them... unless you end up coming back. Even if you don't know you're coming back, you should follow through, it will make your life easier if you do.
2) Be strict... and be patient. Don't let them get away with little things, because they will keep on pushing. Don't take away what you threatened because they're suddenly behaving. But don't show anger, or allow youurself to get angry. You can be strict and nice at the same time.
3) Be fair. In the tougher schools, fairness is everything. If you reward one person for something, reward the others for something else. Same for consequences of negative behaviour... and conseqences need to be consequences. Remind them that THEY have choices to make, and that you did not choose for them. "Jimmy, you have a choice. You can stop being disrepectful, or you can stay in at recess. What is your choice going to be." Give them time to think about it, but not too much time.
4) Don't take it home with you. Vent to someone, go do something fun, let it go, however you can. You're done subbing for the day, and this kind of work likes to hang on to you. Don't let the bad things stay. Most of your classes were good... hold on to that, and remember that even a 14 year old is just a baby. They only think they're all grown up. You are the adult. You need to model good behaviour. Respectful, patient, calm... and true to your word. I can think of so many days I came home upset and frazzled and anxious about facing another day. The funny thing is, it's often not the same ones twice. If one class was rowdy one day, they may be much more settled the next. You just never know, even as a regular teacher.
5) If you love it, hang in there. If you hate it... find another way to make money. It is not worth doing if you don't love it. The effect of it on your emotional and physical health is huge. If you care, the kids will see that, even if you never see them seeing that. If you don't, you shouldn't be doing it. They'll see that too.
Well, having said all of this to you, I wanna say, big hugs! and I hope that you have a less stressful time of it next time. Goodness knows they NEED subs out there, and it is certainly a good trial by fire for regular teaching. A rough class or two is NO reflection on your skills as a teacher... and it will, if you look at it with the right attitude, help you to grow in your skills. Learn from them, and from what they do to you.