If you can see this site from school but have any difficulty actually signing up from a that school location, it may be a filtering issue related to the sign-up security pages. This is being manifested by not being able to see the CAPTCHA image.
You may have to sign up from home or outside of the school network. So sorry!
If you are having any other difficulty signing up, please email steve@hargadon.com with details. Thank you!