My school just started a Pathfinder club at the school after I got a petition to the principal. Because it is in the voluntary "activity block" of our schedules, it will mean that some people unfamiliar to Pathfinder will be there. As I am one of only 2 people in my school who has a good enough grip on the rules to actually DM, that will mean that I will have to get everybody there to "loosen up" to Pathfinder, and to ease them into playing it. How can I do that? The other DM and I will have to teach everybody there Pathfinder, and not scare them away, all in 3 45-minute sessions. There will be about 20 other people in the group. So how can I do that effectively?
Strip down most of Pathfinder
Only introduce a handful of classes and mechanics. You can then introduce them at a later date, but for a first session, if you simply prepare a handful of fighters, rogues, sorcerers and clerics - or something with that sort of spread - you can give people an idea of the 'flavour' of Pathfinder and Fantasy RPGs in general.
More complicated concepts like Vancian casting or less vital parts like skills can be avoided for now, or mentioned in passing then glossed over. It's easiest when teaching a new system to a new player to cover the basics ("This is how you attack", "You've taken damage - subtract 5 from your hp", "You cast a fire spell and kill a goblin") that the players can understand quite easily from video games, then focus on the strengths of PnP RPGs - the interactivity and roleplaying aspects.
Later on, if your players come back for more, you can introduce more classes, add skills, flesh out other subsystems you've mostly ignored, and generally expand the game over time until it's 'proper' Pathfinder, assuming you want to! The group (or you) might enjoy the easier version as much as, or more than, the full version, in which case it's easiest to just leave it as is.
I have had personal experience with running a (lunchtime) RPG club at school. We used 2nd edition D&D, with the only experienced player being a cleric, and then adding a Ranger, Thief, Paladin and Invoker. We only played from level 1 to level 2, but it worked well. We ignored non-weapon proficiencies, I made all the characters beforehand with the other veteran helping me, and a good time was had by all. We never needed to add more of the system later on, because the stripped-down version was quicker (very important with shorter play sessions), and I learned a lot about keeping play moving for a group with players that don't all get on well.
Throw out almost all of the rules.
If you have 45 minutes to teach 20 people how to play pathfinder, you need to show the very basics of the rules, mainly "This is what roleplaying is" and "here's how to see if you succeeded."
Given that ender's game is out and with the success of Harry Potter, I would frame it in HPMOR's army styles. There are groups of 6 "wizards" each (with the extra 2 being... "mercenaries"), set up very much like the story suggests. Instead of "character sheets" they have just the most basic stats. AC, personality, "special abilities", "fire a spell", etc. Have rough approximations of the classes. Make sure there's room for roleplaying (mainly urge them to describe what happens for a bonus to their roll), and move quickly.
This way, there's little actual violence, it's in a setting (magical high school) that they almost certainly can relate to, and there are very few rules getting in the way. If you want, use the basic mechanics of pathfinder to calculate out the numbers.
IF you have the funds available, I would suggest you get a copy of the Pathfinder Roleplaying Game Beginner Box. This is exactly made for beginners, using a simplified subset of the rules.