Gameboy Development Forum

Sadly there's no way to control the sprite priority on the original GB.

Some systems base priority on the address they're placed on in memory or have a attribute for which sprites should overlap which.
The gameboy (non-color models) on the other hand handles this entirely based on the X position of the sprites relative to each other.
They did however solve this on the GBC models by making it based on the address in memory.

Pretty much the only way to give it some form of priority control would be to modify the tiles used by the sprites in realtime by masking out any sprites meant to overlap.
This would however be fairly complex.

PS: If you haven't already you might want to try out joining the #gbdev IRC channel if you have questions.
http://gbdev.gg8.se/irc-channel-gbdev/

http://www.efnet.org/
Just type your prefered nick into the Nickname field and #gbdev into the Channel field on the upper left.