Oh you mean not the Caster Level of the spell inside the Item, rather the prerequisite CL to make generic items in the first place?
hmm ...
I suppose having the appropriate CL in just one school, would allow making an item, but just the one school too.
Otherwise you'd have to have the CL in all schools.
Two possibilities.
**
The fabled Sword of True Strike, doesn't work this way.
To get the +20 to attack requires a CL of 20*3=60, as per the rules.
The younglings always missed that.