Hi Sara

No, you haven't been saying it wrong all these years. You're absolutely right when you say that *"There is six schools in Wales" is incorrect, and that the verb should be the plural "are". What you've picked up on, though, is the common practice of using "there's" for both singular and plural complements. Please read on:

Existential "there" is an unusual subject; it has no inherent number, but takes on the number of the displaced subject:

There were some keys near the safe. (plural with were)
There was a nurse present. (singular with was)

It’s comparable to the relative pronouns "which" and "who", which take on the number of their antecedent ("the guys who were talking" vs "the guy who was talking"). However, in informal style, especially in present tense declaratives with reduced "is", many speakers treat "there" as always singular: they say "There’s a few problems" instead of "There are a few problems". Prescriptivists disapprove, but the usage is too well established to be treated as an occasional slip.

Having said all that, the non-reduced *"There is six schools" is flat wrong and not acceptable, even informally.

PaulM