Minus off that going into NIE is still unknown for now, I would say it was a pretty good experience. I was lucky that my teachers was nice and experienced with both upper and lower sec (I got HODs, SHs, LT). I also saw how teachers handle cases and enjoyed interacting with students.
One advice is to keep a strict face first, let students be scared of you and slowly open up. You will see good teachers that can scold and joke with students. One of my mentor told me to practice your scolding script haha
My supervisor told me the main thing they look out for during lesson observation is class management, engaging the class/problematic students, encouraging discussion.
The rest is the lesson itself. What you can learn now is how to build up to higher order thinking, or what they called scaffolding questions. Being able to do that can add points to your lessons observation. I practiced it through experiences from 1-1 tutoring and when students ask me questions in class (general rule is give them questions that lead to the answers, don't just give them the answer)
Be pro-active, do more is better than do less. Ask teachers for advice for your lesson and try out stuff, but also know that they are busy, so you help me and I help you with consultations, relief lessons etc etc.
Lastly, learn about the school, whether the school is worth to go back when you are trained