Declare as local, really needed?

Try this. Think about how it would work in the two possibilities.

Strict

For Local a = 0 To 13
	Print a + " is it prime? " + isprime(a)
Next
End

Function isprime:Int(number)
	If number < 2 Then Return False
	Local maxfactor = number/2
	For Local factor = 2 To maxfactor
		If Not (number Mod factor) Then Return False
	Next
	Return True
End Function