Before I got into any of your concrete questions, let me just put the suggestion forward to look at WhiteWolf's d10 based Story teller Systems, as in my opinion, they are much less chancy than d20 Systems, and great to easily fit a character concept.
Espacially as they allow to Mix and match skills and attributes Depending on the Situationen and approach to solving the problem. It is also fairly light on math as you normaly just roll the sum of your Attribute and skill in d10 and see how many come up as successes (normaly every die coming up 7+) to determine how successfull they were at their task.
The one Tabletop RPG I played was in this format. I didn't enjoy that game, but not because of the format. The problem is there was only one other person besides the GM, and this was a modern day story where something strange happens, so you're supposed to experience it with your character. But we didn't have enough people for me to really see how players interact together, and it kind of felt like me vs. the GM and his girlfriend, so it was just awkward for me. So, I've played that style before, but didn't fully enjoy it.
I was thinking of a system which is more arbitrary, but where the D20 can give a good idea of how well you succeed, and there could be the tropes of critical fails and critical successes, which could be a lot of fun. I'll consider this system, too, though.
First off, for something going on beyond a One day/afternoon/evening adventure, I would advise against giving the players Set back stories, as in most cases people have a better motivation and enjoyment following the story of the Charakter they created.
What I was thinking is I create several character sheets for them that are essentially partially filled out. For example, one person might be from a well-off Muggle family and grew up around electronics (like smart phones, laptops, xbox). So when they go to the wizarding world and their iphone doesn't work, it gives them an idea of how to react. Or they get a large lump of money at the start of the year, but because their parents can't go to Gringotts they can't deposit more throughout the year, which would change the dynamic from someone who is from a middle-class wizard family who gets a meager allowance for treats throughout the year.
However, the rest of the character is up to them. Are they someone who is used to the power that wealth brings? Are their parents snobs, and they are arrogant as a result? Are they philanthropists who always want to help others? Did their parents pay for private tutors so they are excellent studies? Did their parents pay for private tutors, so they don't know how to study on their own?
I'm basically thinking about creating the lineage and getting them started, but letting them create their own characters.
Also having them meet for the first time at diagon alley, if you want to do an in session sorting, or just have them know before Hand, is a good idea. Instead of planning that out completly though, prepare a Set of smaller Events that take place at a few different locations across it, to allow your players to explore how they like. For example you could have some Animals slip their cages in the pet store, or have one of the professors happen across them as they Buy the books for his/her class.
I really like this idea. So basically I can list the stores, tell them which items are on their school list, and then they can choose what stores to go into, and then I can have various events happen at each? That sounds like a good idea and I'll have to do that.
For the houses, I Honestly suggest going with those you as the GM/ST/sorting hat fits the Charakter best, not what would make for a 'good' Story. If they end up in different houses, you can hark back to their Meeting in diagon alley and probably the hogwarts express as well as just putting them together in most of their classes, seeing as many of them were for two houses at once.
This is what I was planning on doing. If they all fit into the same House, fine. If not, then I can sort them into different Houses. It would be based on their character and their conversation with the Sorting Hat. I would also probably take them into another room when I do the sorting, so they can whisper with the Sorting Hat like they do in the book. I would do them in alphabetical order, too.
Honestly, if you begin changing around teachers, you could start with a different school as well. My suggestion would be to only put in a Different Dada teacher and instead have a seventh grade Wannabe Death Eater cause problems all over the school year.
This is absolutely brilliant. I'll do that. Unless someone has a better idea. Which I doubt.
Edit : as for general advise about GMing, always be ready to scrap all your plans once the players decide to go for something else, and never assume what they will do with the NPCs you provide.
I figured as much. Luckily this will be a school setting, and I will be combination Albus Dumbledore, every professor, Argus Filch, the prefects, Peeves, and all the bad things that can happen. These will be First Year students learning how to play a tabletop RPG.
Edit: To clarify what I mean about the D20 system I was thinking of, it would go something like this:
Say a player has a wand that's good at charms, and they have shown aptitude in that class. They try to use a charm against a weaker wizard in a duel. I take those into account and say either:
*Roll 6 or higher and you succeed
*You have a pretty good chance to succeed, go ahead and roll
*Roll and see what happens (I don't have to tell them everything, do I?)
Before they roll, they can obviously point out things to me, like "remember I just got an A on my test for this specific charm" and I can re-evaluate the success chance and then have them roll. But it wouldn't be like "this gives you +1" or anything like that. I might say "Okay, roll 5 or higher" or "okay, you're more likely to succeed" or "you're already good at charms, don't push it" or "I already counted that."