Is it always like this???

I was asked to sub in seminary this week & silly me, I said yes. On Monday the kids were pretty noisy & I finally had to yell to get their attention as none of them would stop talking. We got the lesson started and most of them paid attention and participated but there were still those who insisted on talking during the entire class, playing games on their phones, walking in and out of the class, etc. On Tuesday they were better and I thought things were going to be okay. Greg & Carinna asked if they could go teach on Wednesday. I told them the ups and downs and that the kids had been really good Tuesday so they should be fine. So they prepared and off they went. Several of the kids just would not stop talking, one of them stood up and decided to write on the board. When Greg asked him to put the chalk down & sit down he refused. Greg ended up taking the chalk from him and asking him again to sit down. Greg asked the kids if they wanted to be there and told them if they did not want to be there to please leave so the rest of them could move on. At first they just looked at him like he was crazy and he told them he was serious, if they did not want to be in class to leave. Several of them got up and walked out. The room settled down and those who stayed ended up having a great lesson. Today when I went back the core group who left came into class and then left to stand in the hall. I went out and asked them if they were coming to class today the two girls said they needed to use the bathroom and the two boys left as well. None of them ended up attending class. As we discussed what had happened the day before they excused it as just being teenagers. They hadn't done anything "that wrong". They blamed Greg & Carinna for not accepting that as teenagers they can't just sit there quietly and listen. The regular teacher has a seating chart as some of the kids are problems if they sit together. Not knowing the kids they of course did not sit where they were supposed to be and then got upset when we called them by the wrong name. One of things I asked today was how many of them had sat in the wrong chair using someone elses box. Almost all of them raised their hands. So all my friends out there who teach or have taught seminary, at what age can kids sit and listen to a 30-40 minute lesson? This class is made up of sophmores & juniors who are 15 & 16 years old, can they really not behave for half an hour?? When did being disrespectful to adults become acceptable and at what point do we as adults step in and say enough? Are we expecting too much by asking them not to talk during class? I'm not talking about answering questions and participating, I'm talking about those who are not paying the slightest bit of attention to the lesson and who are making it difficult for anyone else in the room to pay attention. At what point do you call their parents and ask for help? I have been in primary or relief society for the last few years so I haven't worked with the youth in awhile am I just totally out of touch and it's now "just the way it is" to try and teach over the crowd. Are there no expectations that they can be respectful and at what point do the ones who just refuse to be quiet need to be asked to leave so others can learn? Let me know your thoughts so I know whether or not to ever accept the opportunity to sub again!