If you have Pro you can use the "set sprite priority" command to set the order the sprites are drawn to the screen.
Quote: "SET SPRITE PRIORITY
( Pro )
This command will set the relative priority of the specified sprite. All sprites start with a value of zero giving them equal chance of being drawn last. By setting a single sprite a value of one will cause that sprite to be drawn last. You can specify a unique priority value for each sprite creating an order of drawing for every sprite in your program.
SYNTAX
SET SPRITE PRIORITY Sprite Number, Priority
"
If you don't have Pro the sprite order is the order the sprites are created (the first sprite made will always be under the next one made).