Once . . . . . . . . pupils of . . . . . . . . fifth form read . . . . . . . . book about Robin Hood.

In the first blank, article "the" will be used because article 'the' is used to refer to a definite thing or person and here 'pupils' are definite.
In the second blank, article "the" will be used because 'fifth' is a specific term and "the" is used to indicate specific things.
In the third blank, article "a" will be used because 'book' is a general term or a common noun.