This was a fun read. Thank you for posting it all.
Are you willing to use mods to help with the immersion/set piece scenarios?
MCCC can be used to take the teens out of school and can also slow the game time down so you have time to do the detail such as invited staff before students, etc.
I have one or two 'mass training of sims' setup for different purposes - mine is more Factorio than Harry potter tho :)
To do this I increase the amount of sims allowed in a household as well as the settings above, turn autonomy off and then set the scene up and the let the game roll. Looks great as all the sims are doing what they should. You can also get very precise schedules together doing this
My approach to a day/rsidential period of 'training/school/uni day/etc' is to invite the sims I want to the lot and then shift-click them and use 'Add to Family' (UI Cheats mod). Make sure that when you invite sims from other households to join the family, you leave one sims behind - parents in the case of children/teens or spouse etc. If you want the whole household to undergo training, then keep a few 'house watcher' sims on hand. I have a handful of sims I have created for this purpose (skilled in handiness and usually with the neat trait)
So my performance doesn't tank, I set the amount of sims allowed on a lot and then use that number for adding to my large household (You won't get any walkbys). I find I can get away with about 35 controlled sims, before I start getting lag (5 year old desktop PC).
The sims (ie the trainees) I invite live in their proper homes in normal play and I have to rotate to their proper household and add them back to their proper household once the training is complete. This can take a while if you have a lot of students, but it does give you a chance to tweak the back stories of the students/trainees.
Training institution staff are always resident and when you add in ancillary workers then all your sims slots are mostly gone - hence the increase in the household size. I usually add a cook, caretaker (high handiness), receptionist/greeter/gopher/etc as well as training/teaching staff.
The final mod I use is Lotharihoe's Sims Spawn Overhaul mod - locks all sim walkbys and venue visits to only sims in that world. This means that you can rotate away from your HP setup and play some normal sims for awhile and then come back to it when you feel like playing it again.
It is a bit of a faff to setup, but actually looks more complicated than it actually is and it works brilliantly.
Final tips:
If you want to use uniforms - set them up on a mannequin and then once you have added your sims to the family, they can theno btain the uniform from the mannequin (part of induction process should you want that detail).
You can't directly edit sims in a household larger than 8 sims, but MCCC provides a method for doing this, by loading an individual sim into CAS so you can directly edit them.
Hope the above makes sense, its a bit of a ramble I am afraid. My head is a tadge fuzzy this morn :)
