DBPro free is free to use for individuals only:
Quote: "Note: This is offer is for single home users only. Business and educational establishments cannot use the free version."
Sorry to be the bearer of bad news. You can always stick it on the school's tab.
Other languages similar to DarkBasic? DarkBasic is obviously heavily influenced by QBasic. So, here are two:
FreeBASIC - Modern hybrid of Qbasic and C -style syntax. Looks pretty straightforward and is GPL (free). There's a wikipedia page
here. I've been wanting to try this one out for a while now since it's cross-platform too.
QB64 - Modern Qbasic port. Pretty much Qbasic with extras. I don't know if it actually compiles or just interprets.
Hope that's of some use. If I think of something else I'll post it. Check out wikipedia list of BASIC languages (don't remember where I found it) as each article normally has a code example.