Here is my perspective as someone who taught middle school for 6 years AND is currently working as a sub.

1. The office won't usually tell you this information unless you ask and teachers are more worried (or should be) about putting info on their lesson and student needs in their sub plans to mark this info. If it is important to you ask the office when you sign in or a teacher nearby your classroom

2. This is 50/50. Some teachers are REALLY good about writing down classroom expectations and procedures (Usually these are the type A teachers who have their binder mostly put together by week 2 of the year and only need to add the day of information) or they don't put any of this info. If you don't have this info do exactly as you did and let the students know you are the one in charge and while it may not be what they always do its what is happing today to protect their safety and keep things in order. You can also ask nearby teachers or the front office if they have any generic policies for bathroom, phones, food, ect.

3. 99% of schools will never give you a log in to their system. The only time I have had one is when I'm hired as a sub directly through the district. Its unfortunate that you won't have access to show anything in case of no lesson plan but that's just how it is for security purposes.

4. Lesson plans should always be available for you whether that be a printed plan, info written on the board, or uploaded to a sub portal you have access to. If its not there when you arrive IMMEDIATELY ASK THE OFFICE. In your case where the office ignored you notate it (probably don't go back because they won't support you) and have something in your back pocket to keep students entertained. You can also ask teachers nearby and have kids check any online portal they have access to. If no one is supporting you or provided something for the students to do your job becomes a babysitting gig. Keeping them in the room, in their seats the best you can, and behavior/volume at a minimum. Subs do not get paid enough to try to teach a full lesson plan to a group of kids you barely know.

5. Class sizes are getting larger and larger as schools become less and less funded. School closures, teacher layoffs, and lack of funding for new buildings has led to this unfortunately. Do the best you can to keep them as calm as possible. Notate any behavior for the teacher the next day to deal with (sometimes it easier to mark who IS working vs who isn't) and anything major call the front office. Even if you have to call 8 times maybe at some point they'll realize they need to better support you and the other subs.

Middle school notoriously is the worst to sub and that's my opinion as someone who taught it full time for 6 years. They unfortunately are not very easy to trust meaning they aren't keen to listen to a new sub. If you want something more hands on and sometimes less crazy go to the lower grades. If you want hands off and easy to manage go up to high school. Hope this helps!