You cannot set triggers on tables in the information_schema database.
See manual.
MySQL 5.1 Reference Manual
Chapter 20. INFORMATION_SCHEMA Tables
Usage Notes for the INFORMATION_SCHEMA Database
INFORMATION_SCHEMA is a database within each MySQL instance, the place that stores
information about all the other databases that the MySQL server maintains. The
INFORMATION_SCHEMA database contains several read-only tables. They are actually views, not
base tables, so there are no files associated with them, and you cannot set triggers on them.