Well you're thinking "Oh not another one!".
I can assure you this one stands out, Because i have a sulution to all of them.
Lets get started Chapter One -
First off, Check if Minecraft is blocked at you're school. If not, Skip to chapter Two, If it is blocked, You can try a few things, First try using THIS SITE If that doesn't work, Then when at home, copy your .minecraft from \AppData\Roaming\ Then put it on a USB (Univursal Serial bus)
Then while at school, Press the windows button, Then click on you're name, It will take you to a directory, Then hit "AppData" Then navigate to Roaming then paste .minecraft in there, and run minecraft.exe off a USB.
Chapter Two - If the video cards are out dated
If you run Minecraft and the Video cards are outdated, Follow this
 - Press the windows button, Then right click on "Computer"
 - Then Hit manage, Then navigate to "Display" Then hit "Update Drivers, Using internet"
 - Restart the PC and its done
Load a Linux distro and minecraft onto a USB stick and boot the computer into that. If they allowed USB booting in the BIOS, success! Also, do your homework instead of jerking off all day, you don't want to grow up to be a loser piece of garbage like SOME people I know.