I know how it is to have a dream to make a great game and all,but there are quite some flaws in your plan:
-You said you will make maps in blender=impossible since FPSC collision is horrible for that
-No dynamic entities which means no doors,moveable objects etc.
-Who would ever hack a FPSC game? I mean you can hack with the bugs anyway,you can easily go through any door by just jumping and crouching at it for some time
-Even with the awesome FPSC IP that is comming out soon,the game will be bad,no support for CTF,no dynamic entities,no grenades...
-Im sure atleast 50% of your school mates have a router which makes FPSC games impossible to play,it needs port forwarding for joining and hosting,which will kill the ping
-You are saying your whole high-school will play it...well I think high-school has some proper english class there
Just pick a free MP game like Cube,mod it,make your own MP game there and enjoy it. Much simpler,much less bugs to kill.