How to avoid a select query for holding a Sch-S lock
A Sch-S lock is a Schema stability lock.  It is taken to ensure that the structure of the table doesn't change.  This includes adding/removing columns, etc.  NOLOCK hints and isolation levels affect the locking and versioning of the data in the table, not the structure of the table itself.

Even an online index operation will need to briefly take Sch-S and Sch-M locks.  You cannot avoid this for indexing or schema modification queries.