I am sorry,

I trusted Pinal Dave on this one, without crosschecking MS:

http://blog.sqlauthority.com/2011/02/15/sql-server-lck_m_xxx-wait-type-day-15-of-28/

who wrote

"LCK_M_SCH_S - Occurs when a task is waiting to acquire a Schema Share lock."

an correctly it's

SCH_S <=> Schema stability

SCH_M <=> Schema modification