Can I make a suggestion?
I think your idea is a good one and there is a kernel of something there; however I think that it's too broad in scope. If you're thinking of something that a kid could turn to when the textbook they're using in class is too tough, that's quite a project. In any given subject there are several textbook companies with a book for it, plus multiple editions, etc; so unless a site like yours is very thorough (and you're willing to spend a decade or so working on it), your site won't be very "accessible" to students who struggle.
Also, "everything from calculus to Spanish" is pretty huge. Just looking at something like Spanish or French, you have levels 1-5; and more if you're fortunate enough to be in a district that has a PK-12 language program. Perhaps you could narrow it down to one subject and/or one grade level?
Perhaps your website can be a sort of "knowledge database" for individual classes/schools, that teachers can use in their classes as a teaching tool and then future resource. For example, instead of having "outside experts" write the articles, why not have your website be a kid-friendly Wiki interface where *they* can write the articles? Any class that uses your website will be able to populate the site with information they're learning in class. The teacher is there to help ensure accuracy, etc; and then future students can evaluate/add to/refine it. So it's like a dynamic, digital book report (or science project, or whatever) in perpetuity.
Let's say a class is reading Huck Finn. The teacher tells them what Wiki articles they will need: maybe a literary analysis, an article giving historical context, a Twain bio, etc. Students, individually or in groups, populate the Wiki using their own research from other sources, knowledge they've gleaned from class discussions and reading, etc. Over time the class is continually tweaking and refining the Wiki as their thinking deepens and their knowledge expands. And then they have also created a great study aid for other students around the world.
Basically all you would need to do is design the interface and maybe put together a Teacher's Guide or something.
I don't know what the parameters of your project are, so my suggestions may be useless. However I do think you're going after something inhumanly huge with your initial idea and that will only serve to cause you stress.
Congratulations, by the way, on going for your IB Diploma! That is no small undertaking. You should be proud of yourself! And remember rule #1 when it comes to IB: Don't Panic.
Hope that helps, somehow.