If I start a website with a small amount of usage space, can I add on to it later?

The Montessori Schools are a good choice. You didn't say where you were located but the Bobby Noonan schools are a christian based education. The good thing about them is they offer classes through eighth grade. All these schools are very very costly but worth it if you have the means. Be ready to spend about $700.00 a month for the early years and the tuition goes up as they get older. The amount of students per class depends on the area you live in, the class sizes vary.

I'm not entirely sure what you mean by 'needing more traffic too.' This is assuming by some freak accident I actually /get/ more traffic and need to expand...

And I'll look out for that lol

This all depends but yes as you grew the bigger you will have to expand.

First expand in Bandwidth (from ISP)... Give more users speed to download from your site.

Then expand in Concurrent users to access your site (From Webserver).... More users at same time.

If you grow even more you might need to host two server in a cluster to act as one server...

You would need a Dedicated Hosting service or run your own.... which still means Dedicated.

You will need more traffic too and beware of those advertizing unlimited storage space

apparently its "unlimited" but to a point