First, Freebasic is truly free and runs on several platforms unlike VB that is strictly tied to windows unless you use Wine etc..
Secondly, if you're trying to teach the basics of the basics (variables, if statements etc), an clean environment like the freebasic is much preferable to the visual basic one, as there's no functions, classes and things that are only useful on the vb environment itself like the window editor itself to worry the student and take his time.
Of course, if i was going to use it to teach, would be just the basics, getting the student used into how to formulate his own code and having fun doing so and then quickly boot him into something actually decent like python or C.