1. The size and the length are decided by school rules.
2. The part on which the stick is used is decided by school rules as well.
3. How do you discipline problemed students?
--------------------------------------
Are the expressions grammatical? If not, would you correct them?
1. is fine.
2. would be better rewritten in active voice than passive:
"School rules decide the part on which the stick is used as well."
3. one error, corrected:
"How do you discipline problem students?"
=)