Getting "ORA-00942: table or view does not exist" while table does exist
You created the table in the SYS schema (which you should never, ever do. Really, never).  

When you log in as teacher1 any statement looks for objects in that schema. But there is no TEACHER1.INSTRUCTORS table, because the real name is SYS.INSTRUCTORS (did I mention what a bad idea it is to create objects in the SYS schema?).

You need to run select * from sys.instructors to get access to that table. If you don't want to prefix the table name with the schema, create a synonym in the teacher1 schema: 

create synonym teacher1.instructors for sys.instructors;


Then teacher1 can access the table from the SYS schema without fully qualifying it. 

Again: stop using the SYS or SYSTEM account for anything that is not DBA stuff. Use a regular account for that.