Its good to know that other High school kids are trying to start pathfinder clubs. Last year I ran the first year for myself and a group of friends and I learned a lot. As president of the club I was game master and there were 5-6 kids in the group an only one had role-played before I have some tips that might help you out.
1) Set the club up in a place with a lot of room/big table because you never know who will show up.
2) Be sure to let the players know what they are getting in to (as well as the teachers) - allow bare (absolute, absolute, absolute) minimum of 1 hour to 75 mins to play especially with over 4 players. (with 10+ you WILL need 2+ hrs)
3)With a lot of players it might hep to split into two groups as people tend to get bored when its not their turn
4) NEVER START W/ CHARACTER CREATION this is an easy way to lose people unless they have role played before. The premade characters in the back of the paizo books for 1st lvl work great. Set up a simple doungen crawl with all the stereotypes and something for each players unique skill. Let each player know what each class does.Set up 1-2 sessions just for character creation and take it one class (not person) at a time. When possible refer to the whole group. I found it helpful for no one to play a paladin as the role playing restrictions are to much and he would end up morally obliged to kill the party.
5) Keep it light but discipline is KEY. Last year the players tried to make a mess of everything I did. Keep the planing light and make things up on the fly to keep it fun. After school the last thing people want is a long intro about imaginary places with long names. Keep it short and sweet, with a few laughs to keep people interested. Aim for the kids interests. If they are obsessed with girls have them rescue the princess if they love looking good give cool armor. If a player does something game breaking come down like a hammer or else the whole group will do it.
6) Give the players the bare minimum before they play and explain the rest as it comes up. Explain the die system and what each die is used for. At first all they will need is skill checks and attack/damage rolls. As situations come up let them know about mechanics they can use.
7) The players are new so for the first couple sessions tell them or explain things they might want to do such as check for traps as this will help them to get into the mindset for role playing and prepare them for what will come.
8)Do not plan a whole campaign take it one session at a time. I used the king maker #1 adventure path with, it being sandboxy was perfect for a new group.
9)Most kids play video games so relate it to that as much as possible it helps get the point across especially bioware games.
10)THROW IN A DRAGON they will love it =)
Most of all have fun and keep it light. My experience was that role playing appeals to a lot of people once they try it. With ten people it is essential to split into two groups of 5 with a deputy GM, for the first session do a lecture style to explain stuff, and answer basic questions. If you get their trust and you teach them how to play you will have a lot of fun. Also every once in a while you might want to play another game. I named the club the RPG/gaming club to attract more people. Just be cool with your players and you will do fine.
Best of Wishes