B-A-S-I-C question

You need to make use of the Exit command and have the Return outside of the loop, thus...

Function test()
  z = False
  For i = 0 to 30
    If i = 15 Then z = True:Exit
  Next
  Return z
End Function

You can't jump out of the middle of a loop, as the rest of the program would still think it's in one the next time it gets to a Next - provided Blitz will even compile such a program without an error.

The Exit command is used to jump out of any loop (For..Next, Repeat..Until/Forever and While..Wend).

Hope that helps.