I made custom spells in TES, and I put them in my custom birthsign
In new game, I chose my custom birthsign, but spells in that birthsign doesnt have their own schools
so when I use spells, skill experience doesnt rise up.
how can I set school of them??

You cannot set the school of a spell. The school of the spell is decided by three rules (that I know of)

1) The game doesn't seem to assign a school to a spell which has the "always succeeds" box checked. If that's the case with your spells, then de-select this box.
2) If the game has only one effect or if all the effects are from the same school, then the school of the spell is the same as the school of the effect or effects.
3) If the game has multiple effects from more than one school, the school of the spell matches the effect for which the player's skill is the lowest. So if the spell has two effects from two schools, the school of the spell can actually change as your skill levels change.