Luke, understand, that I am "creatively" challenged, but, I have a suggestion or two. So you know the context I am speaking from, I am using a 15" Macbook Pro, and a Chrome browser. You might want to check it using different browsers on differnt computers and screen sizes. Also, what will it look like on an iPad, etc. Are there plans to have a mobile app for the site?
When I first opened the home page, if I had not know it was about computer science (from your post) I would not have guessed it was about computer science. In my mind, it is more important to let the viewer know exactly what the site is about, and then, who you are.
From a navagation point of view, I believe it would it be better for you to have a "drop down" menue from the tabs at the top (Home, Activities, Competitions, etc) If that is not possible, I would prefer the pages to open in a new tab. That's just me, but I don't like to use the back button. As I looked at the different tabs on the different pages, I would suggest the same thing...drop down menus etc.
On the "Create an account" page, make sure the teachers and kids understand it is FREE to sign up. People like free things!
Visually, each page was very similar in style and color. The style is very boxy, at least to me, and I am not a fan of the color.
Now, on a positive note, I found the content to be good, so make sure you sell it with a more "exciting" look.
Hope my comments are helpful. You have good content and I know students will gain information and skills by using your site.