For a mod I am making, I have an item that teleports a player to spawn, but it can teleport players underground, does anyone have a way to make it so it always teleports players to the surface?

The "spawn point" can actually be a coordinate that is underground.

 

You'll need to write some sort of function that checks for that and returns more suitable coordinates nearby instead of trusting the location of the spawn point.

You only need to get the exact coordinates of the spawn and than call player.setPositionAndUpdate(spawnX, spawnY, spawnZ);