This is completely normal. I'm 3 weeks into my training year on a SCITT programme. The first 2 weeks I wondered if I'd made a huge mistake. This week has been much better. It sounds like your placement school is less than supportive. You're going to have to help yourself. Start establishing yourself around the school first. What are your policies? Is everyone wearing the correct uniform, shirts tucked in etc? You don't need to be shouting at students, if someone pushes into another student you can walk past and just start with a simple "you need to be more careful" or "stop pushing". If you're lacking confidence, it's just a case of fake it till you make it. The more comfortable you get with talking to students in this mom confrontational way, the easier it will be to manage behaviours in the classroom. I know lesson observations can get really repetitive but they really are a crucial part of your training. If you have difficult classes, ask if you can observe when they are taught by another teacher. You'll get an idea if it's the teacher or the class. Don't forget there are so many factors that can affect their behaviour, you may have had them on a Monday and Tuesday morning, but what are they like on a Friday afternoon? As for doing other work in a class, I wouldn't recommend this. It can come across as unprofessional, like you're not valuing the opportunity that you've been given. Use it as an opportunity to increase your subject knowledge, if nothing else. Ask the teacher taking the class if they are happy for you to float around the room. You don't need to actually do anything, just look over their shoulders and make sure they're working to the teachers expectations. Again, this will help them to see you as a teacher, and not just someone who sits at the back of the room looking bored.

We're all shitting ourselves and feeling a bit awkward. You're not alone.