Join conditions and Where conditions in Left Join
Query 1 will show you All records from student with a Class value of 7, and then any related Library_Book records if they exist.

Query 2 will show you all records from student, and related Library_Book records if they exist and if the matching row in student has a Class of 7.

Query 3 will show you All records from student with a Class value of 7, and related Library_Book records if they exist.

I would expect query 1 and query 3 will give you the same results, and query 2 to give you  more rows.

Performance is impossible to talk about without knowing your table structure, distribution of data, indexes, and RDBMS.  I would not expect them to be very different though since your filters are all the same.